Police began walking through the area, blaring whistles, and telling people to “move, move, move!”.

Some, however, seemingly refused to leave - with authorities broadcast messages on large screens telling people of the “immediate evacuation” and urging them to listen to the Secret Service.

The crowd booed in response with others yelling “USA, USA, USA.”

“We are temporarily pausing the event. For your safety please proceed to the nearest exit immediately. Take shelter in your vehicle or a nearby permanent building,” an announcer told attendees.

“Freedom 250 will share updates on programming and doors reopening,” Freedom 250 spokesperson Danielle Alvarez said in a statement that encouraged participants to seek shelter at museums and federal buildings near the National Mall.

Washington’s metro system also said several of its underground stations were available for shelter.

The US Secret Service announced it had temporarily closed checkpoints to screen attendees ahead of Trump’s speech, which was scheduled to begin around 10pm local time.

Military flyovers, which have become a staple of July Fourth celebrations in Washington recently, were also canceled for the rest of the day.

Crowds were building in the area several hours before Trump’s speech.

Severe weather prompted the cancellation of celebrations in Hartford, Connecticut, along with Harrisburg and Wilkes-Barre, Pennsylvania. Spectators at Boston’s fireworks and concert were told to briefly seek shelter before events later resumed.

Plans for fireworks were still moving forward in other cities such as New York, where tall ships passed the Statue of Liberty earlier in the day, recalling the fanfare around America’s 200th anniversary in 1976.