WASHINGTON – Severe weather brought a tornado warning to Washington's suburbs Wednesday, prompting authorities to evacuate thousands of people gathered at the National Mall for a holiday concert and fireworks.

Officers on foot, on bicycles and on horses used megaphones to order the crowds to go into nearby buildings as the sky turned dark and winds picked up, authorities said.

Families packed their blankets and streamed off the grassy area that was to host a concert by the National Symphony Orchestra and fireworks viewing a few hours later.

Police however didn't forcibly remove those who wanted to stay, but no additional visitors were allowed in through any of the 19 security checkpoints, U.S. Park Police spokesman Sgt. Robert Lachance said.

"We planned for this exact scenario, and it went off exactly as we planned," Lachance said.

Authorities said the evening's activities would continue as planned, and expected the crowds to be allowed back around 7:30 p.m. once rain, lightning and hail cleared out.

The evacuated crowds would have to pass back through the checkpoints to get into the area.

The scene was reminiscent of last year, when a severe storm hit the Mall at about the same time and sent people fleeing. Crowds have reached 500,000 people or more in the past.

Security was heightened for the festivities following failed attacks on Glasgow and London last week. Hundreds of emergency responders from about 20 law enforcement agencies were on duty, authorities said.