Niamh Clarke-Willis joined Locog head Lord Coe to hit a button which launched balloons into the sky above the venue for this summer's Games.
Around 40,000 members of the public were at the Olympic Stadium in Stratford to witness the opening, which included a laser light show.
The "2,012 hours to go" event tested the park's management and security.
Lord Coe said: "It's a fantastic feeling. The seven years have just flown by. Tonight is only the start of the story.
"We want thousands of young people to be inspired to take up sport. We hope that for a few of them it will be the start of their journey.
"I'm grateful to everyone who came here tonight to celebrate this moment in history."
Spectators had to queue to enter the venue after airport-style security checks.
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