People participate in the annual Rolling Thunder motorcycle ride to commemorate Memorial Day in Washington D.C., capital of the United States, on May 27, 2018. Hundreds of thousands of motorcycle riders descended on the nation's capital on Sunday for the 31st annual Rolling Thunder Ride for Freedom to honor Memorial Day. (Xinhua/Yang Chenglin)

WASHINGTON, May 27 (Xinhua) -- Hundreds of thousands of motorcycle riders descended on the nation' s capital on Sunday for the 31st annual Rolling Thunder Ride for Freedom to honor Memorial Day.

On this Memorial Day weekend, American flags wave in quiet reverence at cemeteries across the country. The annual Rolling Thunder event in Washington, D.C., which is considered as "the world's largest single-day motorcycle event," gathers those who hope to remember prisoners of war and those missing in action (POW/MIA) in a louder way.

An estimated 400,000 motorcycle riders made the annual trek across the Potomac River on Sunday. The ride began at the Pentagon parking lot around noon and passed across Memorial Bridge, down Constitution Avenue to the Capitol and ended at West Potomac Park, along Independence Avenue, followed by a Rolling Thunder Speakers Program at 1:30 p.m. near the Lincoln Memorial.

Hundreds of fans lined up on both sides of road, cheering and waving American flags to honor the riders as the sea of Rolling Thunder motorcycles rumbled down Memorial Bridge.

Many of the motorbikes fly American flags or the black and white POW/MIA flags that feature the silhouette of a bowed head and the words, "You Are Not Forgotten."

Rolling Thunder was established in 1987 to call attention to prisoners of war or those listed as missing in action. The ride has taken place every year since 1988.