Spectators lined the route from as the riders set off from the Pentagon, making their way to the National Mall.

It had been billed as the last national Rolling Thunder ride by its organizers, but Trump, who loves a parade, appeared to offer a reprieve.

"The Great Patriots of Rolling Thunder WILL be coming back to Washington, D.C. next year, & hopefully for many years to come. It is where they want to be, & where they should be," Trump tweeted from Japan.

"Thank you to our great men & women of the Pentagon for working it out!"

The Pentagon had no comment, referring questions to the White House, which offered no details on what had been worked out.

The current plan is still for this year's ride to be the group's last in Washington, Rolling Thunder president Joe Bean told CNN.

"This is our final ride in Washington, DC. Until we can get into the White House and talk to President Trump and see what he can do for us -- this is our final ride in Washington," the network quoted Bean as saying.

But for Ron Galey, a Navy veteran who served in Vietnam in 1968-69 and has taken part in every Rolling Thunder ride since 1990, Trump's tweet offered hope for future rides.

"Trump said it's not over, so it is not over, and he is the boss of those guys and he keeps his word," he told AFP.

Many pro-Trump banners were visible among the bikers forming up for the parade, a leather-clad, red-white-and-blue tribute to American soldiers missing in the Vietnam War.

The huge motorcycle rally began in 1988 with fewer than 3,000 participants under the motto "We will never forget." The goal was to press for an accounting of those missing in Vietnam.

Over the years, it has grown into a rumbling combination of protest and parade.

- 'Blessing of the bikes'

Riders end up at the Vietnam Veterans Memorial and the nearby Lincoln Memorial for speeches and a concert on the eve of Memorial Day.

Unlike Veterans Day in the United States, which honors all military veterans in November, Memorial Day -- on the last Monday in May -- is aimed specifically at remembering those who died while serving in the US armed forces, were prisoners of war or remain unaccounted for.

The event has the same name as a major US bombing operation against North Vietnam during the war.

Bikers kicked off the weekend of events with a "blessing of the bikes" at Washington National Cathedral on Friday.

Organizers had cited difficult relations with the Pentagon -- where riders line up to begin the rally -- over logistics and costs, in announcing that this year would mark the last national rally.

"As always, the Pentagon is charging us with an outrageous bill for their services," Bean said in a letter to members.

Another letter co-signed by Bean said costs of staging the event had soared to more than $200,000.

"The organization will continue to bring awareness to the public, in years to come, with regional demonstrations," organizers said in a statement on their website.

Trump, who was on an official visit to Japan, offered his support.

"Can't believe that Rolling Thunder would be given a hard time with permits in Washington, D.C. They are great Patriots who I have gotten to know and see in action. They love our Country and love our Flag. If I can help, I will!" he said.

According to the Pentagon's POW/MIA Accounting Agency, more than 82,000 US troops remain unaccounted for in wars as far back as World War II.