By Julien Pretot

BOURG-SAINT-ANDEOL, France (Reuters) - The Tour de France continued in a low-key but defiant mood under heightened security on Friday, with riders, spectators and race officials observing a minute's silence for the scores of victims of the Bastille Day attack in Nice.

A gunman at the wheel of a heavy truck plowed into a crowd celebrating France's national day in the southern city late on Thursday, killing at least 84 people and injuring scores in what President Francois Hollande called a terrorist act.

After race organizers held a crisis meeting with police and officials in the Ardeche region where the 13th stage was to be held, they decided it would go ahead.

"We want this day to be a day of dignity as a tribute to the victims," said race director Christian Prudhomme. "We think, after agreeing with authorities, that the race must continue."

Riders set off at 7:05 a.m. (4.05 a.m. ET), but this time unaccompanied by the usual loud music played to energize the thousands lining the route of what is one of France's most prestigious sporting events.

The Tour's overall leader, Chris Froome, a Briton who lives in Monaco, said: "I just could not believe the images I saw coming from Nice. I do my prep down there. Horrific scenes. Definitely puts things into perspective for us."

That decision to go ahead with Friday's stage was taken "in direct liaison with the highest authorities of the state" Lieutenant-Colonel Eric Luzet, the gendarmerie's liaison officer to the Tour, told Reuters.

"We have 600 officers mobilized to ensure security on this stage. They’re at the start, at the finish and along the route.. After what happened last night in Nice we have changed our security feature accordingly."

Special gendarmerie forces, known as GIGN, have accompanied the three-week race since it started on July 2. The force now has an extra helicopter at their disposal in the event of any threat or attack.

"There was a question this morning whether we could start. And I think it was right to go on racing," said Dutchman Tom Dumoulin, who won the stage, a 37.5-km (23.5-mile) individual time trial from Bourg-saint-Andeol to Vallon Pont-d'Arc in the Ardeche region, some three hours' drive northwest of Nice.

"It’s horrific what happened but we cannot let terrorists decide on how we should be living."

Frenchman Mickael Cherel, one of numerous riders who live and train in the Nice area, said: "We are shocked. I thought of my family because they could have been there, we live 15 km from Nice."

The Tour ends on July 24 in Paris, on the Champs Elysees. Organizers said that cancelling that stage would be 'premature' and that security had been planned three months ago.

(writing by John Stonestreet; editing by Gareth Jones and Robin Pomeroy)