The White House on Wednesday attempted to clarify President Donald Trump's threat the day before to use "heavy force" against "any" protesters at the military parade this weekend in Washington celebrating the Army's 250th anniversary.

"The president supports peaceful protests," press secretary Karoline Leavitt told reporters at a White House briefing after Trump on Tuesday did not distinguish between peaceful and violent protesters.

"He supports the First Amendment. He supports the right of Americans to make their voices heard," she added, after being asked what Trump would allow at the parade given his military response to the protests against his immigration policies in Los Angeles. "He does not support violence of any kind. He does not support assaulting law enforcement officers who are simply trying to do their job."

"It's very clear for the president what he supports and what he does not," she said. "Unfortunately for Democrats, that line is not been made clear, and they've allowed this unrest in this violence to continue, and the president has had to step in."

The president's comments on Tuesday said protesters would be "met with heavy force" if they arrived in Washington for the parade, which occurs days after he sent the National Guard and the Marines to Los Angeles to quell protests against operations conducted by U.S. Immigration and Customs Enforcement.

"These are people who hate our country," he said in the Oval Office.

"We're going to celebrate big on Saturday," Trump added. "If any protesters want to come out, they will be met with very big force."

U.S. Secret Service and local D.C. officials have said they only expect several small protests at Saturday's parade, and Trump himself on Tuesday night appeared to soften his earlier Oval Office comments somewhat, saying, "As long as we have the military there, the protests won't mean anything."

"The military will be very heavy force -- very proud to tell you that," he told reporters who had asked him what he meant. "They might as well turn around. They're wasting their time."

Tall fencing has been set up and other security measures have been taken around Washington in the lead-up to the parade -- measures the White House said are purely "proactive" and not in reaction to the protests in Los Angeles.

"These are proactive security measures to protect those marching in the parade, many of whom will be veterans, and our brave men and women in uniform and Gold Star families," Leavitt said. "And of course, it's to protect the spectators who will be enjoying this incredibly patriotic show on Saturday."

Leavitt's comments came as the first soldiers arrive in the district for the parade.

Approximately 6,700 soldiers will participate, and there will be eight marching bands, 24 horses, two mules and a dog.

Dozens of tanks, military vehicles, howitzers and various aircraft, including those used in World War II and the Korean and Vietnam wars, will be on display, and the Army on Wednesday added rocket launchers and precision-guided missiles to the festivities.

The White House also made a last-minute request for the Air Force's Thunderbirds to fly over the parade, according to a U.S. official.

The event has grown considerably in size in recent months, with a parade added after Trump's inauguration.

"The President wants the Army Birthday Parade to feature the strength, talent, and creativity of all our military servicemembers," White House spokesperson Anna Kelly said. "The Thunderbirds flyover will inspire patriotism and awe for all who attend!"

ABC News' Anne Flaherty contributed to this report.