Federal officials ordered thousands of people to evacuate the National Mall in Washington on Saturday night ahead of a presidential speech and fireworks display to celebrate the nation’s 250th birthday. Crowds were told to seek shelter as a line of severe storms approached the city.

Park Police and other law enforcement officers started urging people to leave at about 7 p.m., just as the festivities were expected to start. Many attendees had stood in hourlong security lines in blistering heat, waiting to go through Secret Service metal detectors to get to the mall. And many of them refused to leave.

After lightning was spotted in the area, spectators were told to take shelter in nearby buildings, including Smithsonian museums, the Jefferson Memorial and several designated federal departments, including the Internal Revenue Service. But some of those buildings were not open to serve as shelters.

People crowded around the entrances of the Commerce Department, locked out. A nearby officer shrugged when someone asked how to get inside.

In a statement provided a little after 7:30 p.m., a spokesman for the Secret Service said that no one else was being admitted to the mall, and that “this action was taken solely in the interest of public safety.” It added, “We have no estimate for when screening may resume.”

Some people on the mall became angry, with several hundred refusing to leave and arguing with Secret Service personnel, chanting, “U.S.A.! U.S.A.!” A federal law enforcement officer tried to assure people that the event had not been canceled, though many were not convinced.

A severe thunderstorm watch from the National Weather Service remained in effect until 10 p.m. for the Washington region.