Chris Froome had to be prompted by a BBC journalist to register for the Olympic road race moments before the start at Copacabana beach.
The three-time Tour de France champion, seeking to join Bradley Wiggins as the only Tour winner to win Olympic gold in the same year, was unaware his name was being called over the public address system.
Riders must sign in before races, but Froome had forgotten until the journalist shouted over the barrier.
The British team of Froome, Steve Cummings, Geraint Thomas, Adam Yates and Ian Stannard were on the front of the peloton as they rolled over the start line for the 237.5-kilometres race.
The race began by rolling towards the first of four laps of the 24.7km Grumari Circuit, which features two punchy climbs.
Two laps and a loop of the 25.5km Vista Chinesa Circuit, which features the steeper ascents, follows.
There will be a descent to the finish at Copacabana, which could be decisive.
Germany listed track sprinter Maximilian Levy in the team, exploiting a loophole in the qualification criteria, and he rode 33km before quitting the race.
Tom Dumoulin had stepped off his bike earlier than that.
The Dutchman, who won two stages, quit after 12km as he is prioritising Wednesday's time-trial, which Froome is also targeting.
Germany also had a rider in the early break, Simon Geschke, a Tour stage winner.
Also in the six-man escape group was Jarlinson Pantano of Colombia, another Tour stage winner, and Michal Kwiatkowski of Poland, a Team Sky colleague of Froome's.