Armed with plenty of black Sharpies, President Donald Trump shattered the record for most executive orders signed on his first day in office and became the only commander-in-chief to partly do so in an arena packed with thousands of people.

But the nation's 47th president is already facing legal challenges to some of his most controversial executive actions and others have stoked outrage.

"With these actions, we will begin the complete restoration of America and the revolution of common sense," Trump said during his inauguration speech on Monday. "It's all about common sense."

With those words, Trump immediately began rescinding numerous executive actions taken by now-former President Joe Biden, alleging in a written preamble published with his presidential actions that "the previous administration has embedded deeply unpopular, inflationary, illegal, and radical practices within every agency and office of the Federal Government."

Trump said his executive actions represent his administration's priorities of protecting the southern U.S. border, reforming and improving government bureaucracy and unleashing American energy by ending Biden’s policies of climate and streamlining permitting that he contends has stymied energy production. Another goal, he said, is to protect women from "radical gender ideology" and recognize only two sexes, male and female.

"As of today, it will henceforth be the official policy of the United States government that there are only two genders: male and female," Trump said in his inauguration speech.

In another unprecedented move, Trump signed his first eight executive actions in front of a wildly enthusiastic crowd filling Washington D.C.'s 20,000-seat Capital One Arena, capping his inauguration parade.

"President Trump has really made political theater a core part of how he uses unilateral power," Jon Rogowski, an American politics professor at the University of Chicago, told ABC News. "He wants to make sure people know he has taken action that’s consistent with the kinds of campaign promises that he made when he was running for office."

What is an executive order?

Executive orders are signed, official documents through which the president manages the operations of the federal government and provides instructions to the executive branch about how to interpret the law, Allan Lichtman, a professor of history at American University in Washington D.C., told ABC News.

"An executive order is a directive issued by the president that goes into the Federal Register. It has the force of law, but it does not require an act of Congress," Lichtman said. "Although it has the force of law, it can be repealed by a subsequent president issuing executive orders of his or her own."

Lichtman said that unlike laws passed by Congress, executive orders are more vulnerable to challenges in the courts, either substantive or procedural.

"The reason why we are seeing the government by executive order is twofold: Donald Trump was singularly ineffective in getting his agenda through Congress during his first term, even with Republican majorities in the House and the Senate during his first two years. The second reason is they have such a thin majority in the House," Lichtman said.

In the past, executive orders were typically challenged after the federal government took action to implement them and someone claimed they had been harmed by that executive action, Rogowski told ABC News.

"Now, it has changed. Presidents are being sued immediately after they sign an executive order by organizations who represent collections of individuals or by states' attorneys general," Rogowski said.

Rogowski said that during Trump's first term “it became more common for groups and for states to directly challenge the president immediately upon the issuance of the directive with which they disagreed.”

"Over the last decade, there’s been more organization on the ground and among legal groups and states to put pressure on the presidential administration’s use of executive action," Rogowski said.

Trump's actions prompt legal challenges

Trump on Monday granted 1,500 people convicted of crimes stemming from the Jan. 6, 2021, insurrection at the U.S. Capitol, "a full, complete and unconditional pardon" and commuted the sentences of 14 others involved in the riot. Trump called them "hostages."

The Constitution gives a sitting president the power to grant clemency to just about anybody and such actions cannot be reversed. Before leaving office on Monday, Biden issued a slew of preemptive pardons to potential targets of the incoming Trump administration, including several close relatives, Dr. Anthony Fauci, retired Gen. Mark Milley and lawmakers who served on the House Jan. 6 committee, including former Republican Rep. Liz Cheney.

"What they have done to these people is outrageous," Trump said while signing the pardons and commutations in the Oval Office.

The pardons and commutations immediately sparked a backlash from both Democrats, Republicans and the union representing members of the U.S. Capitol Police.

"The president's actions are an outrageous insult to our justice system and the heroes who suffered physical scars and emotional trauma as they protected the Capitol, the Congress and the Constitution," former House Speaker Nancy Pelosi, D-Calif., said in a statement.

Former Rep. John Katko, R-N.Y., an ABC News contributor, called the pardons of the Jan. 6 rioters "unsettling" to members of Congress on both sides of the aisle.

"They weren't 'hostages,' they were defendants. I was there," Katko told ABC News Live Prime anchor Linsey Davis on Monday night. "A lot of people in the Capitol were beaten. I saw a lot of officers that were severely beaten and injured that night. It was a bad scene."

The National Fraternal Order of Police, the union representing U.S. Capitol Police officers, released a statement, saying, "The vast majority of Americans do not support letting criminals off scott-free or being given lighter sentences when they’ve attacked law enforcement officers."

Another controversial executive order Trump signed was one aiming to cut off birthright citizenship. Critics immediately pounced on Trump, arguing people born in the United States are granted citizenship under the U.S. Constitution's 14th Amendment even if their birth parents migrated here illegally.

Asked by reporters if he expected the order to prompt legal challenges, Trump responded, "We'll see. We have very good grounds. People have wanted to do this for decades."

Five federal lawsuits were filed already this week, challenging Trump's birthright citizenship executive order. One was filed in Massachusetts federal court by 19 Democratic state attorneys general, the city of San Francisco and the District of Columbia, accusing Trump of seeking to eliminate a "well-established and longstanding Constitutional principle."

A similar federal lawsuit was filed by the states of Arizona, Oregon, Washington and Illinois. The American Civil Liberties Union also backed another lawsuit filed in New Hampshire asking the court to block Trump's order.

"We will not let this attack on newborns and future generations of Americans go unchallenged. The Trump administration's overreach is so egregious that we are confident we will ultimately prevail,” said Anthony D. Romero, executive director of the ACLU.

A separate lawsuit filed in Massachusetts U.S. District Court was brought by an undocumented expectant mother and two nonprofit groups, arguing the executive order is an attempt to reinterpret the 14th Amendment's guarantee of citizenship to nearly every person born in the United States.

A fifth lawsuit was filed late Tuesday in U.S. District Court in Maryland on behalf of two nonprofit groups, five pregnant undocumented women and their future children.

Mexico opposes Trump's actions

In other executive actions, Trump issued a proclamation declaring a national emergency at the southern U.S. border, saying, "America’s sovereignty is under attack."

"Our southern border is overrun by cartels, criminal gangs, known terrorists, human traffickers, smugglers, unvetted military-age males from foreign adversaries, and illicit narcotics that harm Americans, including America," the proclamation reads. "This invasion has caused widespread chaos and suffering in our country over the last 4 years."

Trump also issued an executive order designating certain cartels as foreign terrorists. He signed yet another executive order changing the name of the Gulf of Mexico to the Gulf of America.

Many of the proposed executive orders require help from international partners like Mexico and would almost certainly spark legal battles.

"We have to relate as equals, not subordinates," Mexican President Claudia Sheinbaum said Tuesday in reaction to Trump's executive orders pertaining to the southern border.

Sheinbaum said her focus is on defending Mexicans and her country's Constitution.

"You have to keep a cool head with what he signed," Sheinbaum said. "You have to read the decrees. In fact, we continue to study them because some of them went out very late at night and we have a team working on that."

Sheinbaum said some of Trump's executive actions are similar to ones he signed during his first term, including the one proclaiming a national emergency at the southern border.

"There was cooperation between the two governments at that time," Sheinbaum said. "When President Biden comes in, he removes it and now President Trump reinstates it."

Sheinbaum said Trump's executive action to prevent asylum seekers from entering the U.S. from Mexico is almost identical to the decree he made in 2018.

As far as changing the name of the Gulf of Mexico, Sheinbaum said, "For us and the world, it is still the Gulf of Mexico."

"We will always act in the defense of our independence, the defense of our fellow nationals living in the U.S.," Sheinbaum said. "We act within the framework of our constitution and laws. We always act with a cool head."

Legal action could delay implementation of orders

Rogowski said legal challenges could delay the executive orders from being put in place.

“I don’t want to undermine the chances that any of these could have real, tangible consequences for people, but I suspect Trump is going to find it frustrating that many of his actions that he intends to have an immediate effect are instead going to be held up in court for weeks, months or even years, in some cases," Rogowski said.

During his first term, Trump signed an executive order that banned foreign nationals from seven predominantly Muslim countries from entering the United States for 90 days, suspended entry to the country of all Syrian refugees indefinitely, and prohibited any other refugees from coming into the country for 120 days. Rogowski noted that legal challenges stymied the so-called Muslim ban for 10 months and forced Trump to issue two revised versions of the ban before the U.S. Supreme Court upheld it in a 5-3 ruling issued in June 2018.

'A lot of them are symbolic'

Trump also issued an executive order withdrawing the United States from the World Health Organization, alleging the organization mishandled the COVID-19 pandemic.

As he did in his first term, Trump signed an executive order removing the United States from the Paris Agreement, an international treaty to mitigate climate change that was enacted in 2016. Trump said the agreement doesn't reflect U.S. values and directs "American taxpayer dollars to countries that do not require, or merit, financial assistance in the interests of the American people."

Rogowski said some of the executive actions taken by Trump, like changing the name of the Gulf of Mexico and restoring the name of America's 25th president, William McKinley, to the highest peak in North America, are primarily "symbolic."

Renaming the Gulf of Mexico to the Gulf of America "is not going to advance any of Trump's economic goals or priorities that he outlines, other than make a symbolic change," Rogowski said.

Lichtman agreed. "They're symbolic in that they resonate with themes that Trump has been professing ever since he arrived in presidential politics a decade ago," he said.

But Lichtman added that many of Trump's executive orders "are real and they have a real impact on the American people."