James Cracknell was the first celebrity over the finish line with a time of just over four hours and 36 minutes.
"It was brilliant," he said afterwards.
"Apart from my time, the best thing was the fact that there were thousands of people flooding into the Olympic Park, not to watch something, or see top performers, but to do something themselves.
"If there are any legacy discussions going on, that is the selling point for me - nearly 20,000 people riding out for 100 miles. I can really see this being like the London Marathon."
London mayor Boris Johnson praised the "wonderful crowd" who spurred him on to complete a gruelling 100-mile road race in the capital.
The event - part of a weekend described as the UK's largest festival of cycling - also saw famous faces including Rugby World Cup winner Matt Dawson, two-time Olympic rowing champion James Cracknell and Olympic champion sprint hurdler Sally Gunnell take to the saddle alongside some 16,500 others.
Speaking on the finish line on The Mall, Mr Johnson said: "This is a great day for cycling, and for this city.
"I want to thank the organisers and the wonderful crowd who made all the difference as we cycled by, helping to spur me on to finish the gruelling 100 miles in a respectable eight hours.
"Huge congratulations to everyone who took part - RideLondon is now firmly on the map as one of the greatest cycling events in the world."
Cyclists have begun racing across the capital as part of the Prudential RideLondon weekend, in one four separate events designed to appeal to riders of all ages and abilities.
Around 50,000 people are taking part in a mass event that saw many of the capital's roads closed to traffic yesterday.
London mayor Boris Johnson was among thousands of others taking part in a 100-mile road race today.
Rugby World Cup winner Matt Dawson has tweeted a picture of himself alongside Olympic rowing star James Cracknell, saying "Here we go. There's no point competing with this fella."
London Mayor Boris Johnson has been wheeled out throughout the year in promotion of the RideLondon cycling festival.
The regular rider will take part in the 100-mile challenge on Sunday.
"My team has told me I will never make it," Mr Johnson said. "They think I am too clapped out to cycle 100 miles but the more they discourage me, the more determined I am to have a crack."
Riders of all ages and abilities will navigate roads around London and Surrey this weekend as part of the UK's largest festival of cycling.
More than 70,000 are expected to get on their bikes for the events at the Prudential RideLondon festival.
The 100 mile London-Surrey, starting at the Olympic Park and finishing on The Mall, is set to be among the highlights.
Up to 20,000 amateur cyclists, including London Mayor Boris Johnson, will be taking part.