Washington prepares for Trump's military parade with 18 miles of fencing and 175 magnetometers

Army officials have estimated around 200,000 attendees for the evening military parade, and McCool said he was prepared for "hundreds of thousands" of people.

"We have a ton of magnetometers," he said. "If a million people show up, then we're going to have some lines."

A total of 175 magnetometers would be used at security checkpoints controlling access to the daytime birthday festival and the nighttime parade. Metropolitan Police Department chief Pamela Smith predicted "major impacts to traffic" and advised attendees to arrive early and consider forgoing cars for the Metro.

"This is a significant event with a large footprint," she said. "We're relying on the public to be an extra set of ears and eyes for us."

The military parade has been designated a National Special Security Event — similar to a presidential inauguration or state funeral. That status is reserved for events that draw large crowds and potential mass protests. It calls for an enhanced degree of high-level coordination among Washington officials, the FBI, Capitol Police and Washington's National Guard contingent — with the Secret Service taking the lead.

The Army birthday celebration had already been planned for months. But earlier this spring, Trump announced his intention to transform the event — which coincides with his 79th birthday — into a massive military parade complete with 60-ton M1 Abrams battle tanks and Paladin self-propelled howitzers rolling through the city streets.

Multiple counter-protests of varying sizes are planned for Saturday, with the largest being a mass march to the White House. Officials say they are also on alert for signs that the immigration-related clashes between law enforcement and protesters currently roiling Los Angeles would spread.

"We're paying attention, obviously, to what is happening there. We'll be ready," McCool said. "We have a robust plan for civil disobedience."

Agent Phillip Bates of the FBI's Washington Field office, which is tasked with counterterrorism and crisis management, said there were "no credible threats" to the event at the moment.

Lindsey Appiah, the deputy mayor for public safety, told The Associated Press last week that the city had long-standing plans for the Army birthday celebration. But those plans "got a lot bigger on short notice" when Trump got involved.

Still, Appiah said the city has grown "very flexible, very nimble" at rolling with these sort of changes.