Great Britain's rowing pair Heather Stanning and Helen Glover ended the host nation's five-day wait for Olympic gold and dedicated their success to the "whole country".
In front of 30,000 fans at Eton Dorney, they produced a blistering display to win the women's pair.
Glover told BBC Radio 5 live: "This was for the whole of the team and the whole of the country."
Bradley Wiggins has started his bid for gold in cycling men's time trial.
Forty minutes after Glover and Stanning's triumph, Britain's men's eight boat clinched bronze to take Britain's medal tally to six.
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Helen Glover and Heather Stanning
Glover & Stanning win first GB gold
Glover and Stanning led from start to finish, becoming the first British female rowers to win an Olympic gold.
Stanning has taken time out from her army career to compete in the Olympics. She said: "I want to collapse I'm so overjoyed. We hope this is the start of things to come from the Great Britain team.
"I'm probably talking rubbish now. We've got out ahead throughout the season and we wanted to do that today, and not give anything back. Helen was telling me to keep on going. Thanks for all the support in Afghanistan, I'm so proud to be associated with you."
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I'm delighted about Heather and Helen's gold medal in the rowing - an absolutely fantastic effort
David CameronPrime Minister
PE teacher Glover added: "I really hope my story can be an inspiration for kids in PE watching this, or at home thinking about taking up a new sport. Just go on, go for it, you don't know what's going to happen."
Prime Minister David Cameron was on a visit to the Giant's Causeway in Northern Ireland. He said: "I'm delighted about Heather and Helen's gold medal in the rowing - an absolutely fantastic effort."
Mr Cameron spoke after he visited the famous coastal causeway, where one of the stones, worn away by visitors who sit in it for good luck, is known as the wishing chair.
He continued: "I was sitting in the wishing chair, I was told I could make my wish. I won't say what it was, but as soon as I got back to my mobile phone I had the good news. Fantastic news, well done to them, a great success for the United Kingdom team."
Mayor of London Boris Johnson said Glover and Stanning's triumph would be celebrated throughout the country
"Not only have they achieved the first gold medal for Great Britain at London's Games, they have bagged the first-ever rowing medal for Team GB's women at an Olympic Games," he said.
"It is an outstanding achievement and I am sure it will spur on all our athletes in the medal stakes. It is fantastic - go Team GB."
In the men's eight, Great Britain claimed bronze medal as world champions Germany took gold in a thrilling final.
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Great Britain and Germany men's eight rowers
GB get bronze as Germany win men's eight
The crew, featuring 1992 gold medallist Greg Searle, pulled ahead of Germany at halfway, but ran out steam with Canada pipping them on the line for silver.
After near-misses in cycling and equestrian, Great Britain have more gold medal chances on what could turn out to be a wonderful Wednesday.
Tour de France champion Wiggins is attempting to become Britain's most-decorated Olympian. A medal of any sort would take him past Sir Steve Redgrave's tally of six Olympic medals.
And Team GB could taste more glory in the Aquatic Centre when Michael Jamieson and Andrew Willis compete in the men's 200m breaststroke final after qualifying first and third from the semi-final.
There was disappointment for Britain in the women's time trial as Beijing silver-medallist Emma Pooley finished sixth and Lizzie Armitstead, who took road race silver on Sunday, came 10th.
Elsewhere, eight Badminton players were disqualified from the women's doubles competition for "not using one's best efforts to win".
The four pairs were accused of wanting to lose in an attempt to manipulate the draw for the knockout stage.