Thousands of motorcycle riders poured into Washington, D.C., on Sunday to pay tribute to military veterans, prisoners of war and soldiers missing in action at the annual Rolling Thunder Motorcycle Procession. 

Motorcyclists have been gathering at the National Mall in downtown Washington since 1988 to celebrate Rolling Thunder, an event where bikers ride into D.C. to honor prisoners of war and those missing in action from the Vietnam War, reports DoD News.

The bikers gathered at the Pentagon in Virginia, where they started the "Ride for Freedom," and then crossed the Potomac River over the Memorial Bridge and circled around the National Mall.

According to Rolling Thunder spokeswoman Nancy Regg, more than 1 million bikers and spectators were estimated to have been in attendance, making the event the largest one-day motorcycle gathering in the world.

"They're not here to party and get drunk," Regg said speaking of Rolling Thunder, reports Reuters. "No fights. No attitudes."

She added that there were no reported criminal incidents at the event.

Supporters at the event cheered and waved as the bikers throttled their engines and waved back, with either U.S., Prisoner of War or Confederate flags.

Rolling Thunder was held just one week after a deadly fight between rival motorcycle gang members at broke out at the Twin Peaks restaurant in Waco, Texas.

Police say that the shooting occurred around noon on Sunday after two rival biker gang members got into a confrontation over a parking space inside the restaurant's bathroom, reports CBS Dallas / Fort Worth. Their fight then moved to the restaurant's bar, where more people jumped in and began stabbing each other. Eventually the ruckus, which involved as many as five Texas biker groups, moved outside to the parking lot, where several people opened fire.

The brawl left nine suspected gang members dead, while 18 others were hospitalized. Police say that 170 bikers have since been taken into custody on charges of engaging in organized crime. Plus, authorities recovered more than 100 weapons at the crime scene.