It's hog heaven in the nation's capital.

For the 21st year in a row, Rolling Thunder roared into Washington, D.C., on Sunday for its annual veterans tribute, bringing together an estimated 350,000 motorcyclists — along with thousands of activists, fans and spectators.

Bikers from the group's 88 chapters —across the country and overseas— came together to bring attention to U.S. service members held captive or missing in action.

Riders took off on their rumbling "Ride For Freedom," driving from the Pentagon, across the Potomac River by way of the Memorial Bridge and on to the Vietnam Veterans Memorial.

The party-like atmosphere was punctuated by speeches, tributes and music. Actor John Amos, who read Gen. Colin Powell's "A Letter to a Soldier" lent his voice to the cause.

"This is the most important gathering I have ever been apart of. I share their convictions that all these men and woman must be accounted for. We live in the greatest country in the world, and a gathering like this confirms that we have have more freedom," Amos said.

President Bush also met with some of the group's organizers, and described the parade of motorcycles — many adorned with the American flag — as "a magnificent sight. "I am just so honored, and I want to thank you and all your comrades for being patriotic and loving our country as much as you do," said Bush.

Organizers then presented President Bush with a leather biker's vest and cap.

Rolling Thunder 21, along with honoring the families of fallen and missing service members, are advocating for the fair treatment of veterans returning from the wars in Iraq and Afghanistan.

The Associated Press contributed to this report.