WASHINGTON (Reuters) - Hundreds of trucks, recreational vehicles and cars were circling the outskirts of Washington on Sunday, threatening to cause traffic backups around the capital as part of a protest against COVID-19 restrictions.

The so-called "People's Convoy," which originated in California and has drawn participants from around the country, is calling for an end to all pandemic-related restrictions. It was inspired by demonstrations last month that paralyzed Ottawa, Canada's capital city.

The convoy's message has been undercut in recent weeks as major U.S. cities have rolled back mask mandates and other measures against COVID-19. President Joe Biden, a Democrat, signaled in his State of the Union speech on Tuesday that the country was entering a new, more controlled phase of the pandemic without business lockdowns or school closures.

But that did not stop hundreds of vehicles from gathering on Friday and Saturday at the Hagerstown Speedway, a racetrack in Maryland about 80 miles (129 km) northwest of downtown Washington.

On Sunday morning, many left in convoy to drive slow laps around the Beltway, a highway that encircles the city. They honked their horns as they set off, while onlookers waved American flags, according to a Reuters witness.

It remained unclear whether the convoy would drive into downtown Washington. Organizers said the plan for Sunday was to stay on the Beltway then head back to Hagerstown.

At the racetrack on Friday night, one participant who described himself as the lead trucker told a cheering crowd he would drive his truck into the heart of the American capital.

"D.C., the government, whomever, can claim that they have all this opposition for us waiting in D.C.," the man said. "But that flag on the back of my truck will go down to Constitution Avenue between the White House and the Washington Monument."

Federal law enforcement agencies have been coordinating with state and local authorities for weeks in preparation for the possible arrival of the convoy, according to one U.S. official who requested anonymity to discuss internal operations.

A February 26 U.S. Department of Homeland Security (DHS) bulletin to law enforcement reviewed by Reuters said trucker convoys could hinder emergency responders depending on the size of the protest.

(Reporting by Julio-Cesar Chavez, Gabriella Borter and Ted Hesson; Editing by Daniel Wallis)