Britain's Laura Trott won a superb second gold medal of the Games with victory in the women's omnium.
Having claimed gold in the women's team pursuit on Saturday,
she is now a double Olympic and world champion at just 20 years of age.
Trott delighted with second gold medal
"I can't believe this has happened to me," said Trott, who finished with a total of 18 points.
"I'm peaking at the right time and it's all thanks to the coaches, and the support of the crowd and my family."
The overnight leader in the multi-discipline event, Trott dropped back into second place behind American Sarah Hammer in Tuesday's opening session.
Hammer stretched her lead to two points in the penultimate event, the scratch race, but Trott produced an Olympic record for the 500m time trial to snatch the gold.
That relegated 28-year-old Hammer to the second step of the podium, with Australia's Annette Edmondson taking the bronze.
2012 track cycling golds
Britain have won seven out of 10 track cycling gold medals at the London 2012 Olympics
With the winner of each event in track cycling's version of the decathlon getting one point, the second-placed rider scoring two points and so on, Trott needed to beat Hammer by three clear places to win the inaugural women's Olympic omnium title.
That is what happened at the World Championships in Melbourne in April,
but so close were the two riders that many suspected the result here would be a tie, meaning the medal would be decided by the cumulative total of their three timed rides, the 250m flying lap, the 3,000m individual pursuit and the 500m time trial. That would give Hammer a 0.681 second advantage going into the time trial.
However, when French sprinter Clara Sanchez laid down an early marker of 35.451 it looked like Hammer was going to have to go considerably quicker than ever before, with Trott, a more accomplished sprinter, favourite to win the final discipline.
Edmondson's 35.140 then gave Trott the opportunity to put three places between herself and Hammer - the leading pair the last to ride - and she did not disappoint the raucous velodrome crowd. Her 35.110 was another Olympic record for the dominant British track cycling team.
"I came into today not feeling as good as yesterday and to finish off with that last 500m is unbelievable," added Trott, who won three of the six events, was second in another and third in a fifth.
She has now won eight gold medals from eight opportunities at the three major championships she has been to.