Lord Coe has said the London 2012 Olympic team is "deeply shocked and saddened" by the terrorist explosions which rocked the capital city.
Bid leader Coe gave a statement before flying back to the UK on Thursday from the Olympic vote in Singapore.
"Our thoughts are with the families of the bereaved and all those who have been injured in these despicable terrorist attacks," he said.
London beat Paris, Madrid, New York and Moscow to win the 2012 Games.
Before the attacks, Coe's team had been celebrating a surprise victory.
But the mood was shattered as officials prepared for departure back to the UK after the International Olympic Committee vote.
Plans for a victory homecoming, and news conference, had however already been cancelled.
IOC officials have said the attacks will not affect the decision to award the Games to London.
Before the attacks, Coe had indicated his intention to start work quickly on putting plans for the Olympics into practice.
"There's now real hard graft ahead over the next seven years, and particularly the next six months," said Coe.
Coe was keen to avoid the sort of slow build-up which left Greece scrambling to get ready in time for the Athens Games last year.
"Over the next 100 days there are some pretty serious time-lines to meet," he said at his final news conference in Singapore.
"I think we've learned pretty well from previous Games and previous organisations that we really do have to start early.
"We have a natural advantage because the funding and planning is in place, and work has already started on some of the venues.
"Our responsibility is to make sure that the additional venues that need building get built, that the infrastructure comes in on time, and that we have a structure that allows us to drive this every day for the next seven years."
London faces a number of key challenges as the seven-year countdown to the Olympics starts in earnest.
This will be an obvious concern following the terrorist attacks that struck London within 24 hours of the city winning the Olympic vote. A projected £200m was set to be spent on security.
Sources of income include a special lottery game to help cover projected costs of £1.5bn to run the Games and £560m to construct the new venues.
The bid team have promised that hotel prices will be fixed. At present there are 100,000 hotel rooms in the capital, with another 17,000 expected by 2012.
The bid leaders have already warned against complacency, having seen other cities slacken off in their efforts after winning the vote, and plan to start construction almost immediately.
An improved rail service is at the heart of the plans and the Government have pledged £7bn to help fund better travel links.
Coe's priorities include setting up and appointing members of the organising committee for the Games (LOCOG), which he will head.
Coe said he was not sure if he would still be running LOCOG by 2012, but that he would still be involved in some way.
"Who knows? Seven years is a long, long time.
"I have agreed to set off on the journey and I want to be involved in any way I can make a contribution - I'm not going back into politics though, I've done that!"
Coe must finalise details of an Olympic bill to go before Parliament next week and make sure the new Olympic lottery, which will contribute part of the £2.375bn budget for the Games, is launched later this month.
"It is just the most fantastic opportunity to do everything that we've always ever dreamed of in British sport," said Coe.
"We now have the chance over the next seven years, and way beyond that, to just change the face of sport and just get what I've always, always wanted - more young people into sport."
UK Athletics chief executive David Moorcroft said the victory would leave a lasting legacy.
"It's the best possible news for the young people," he told BBC News 24.
"I think they can realise their Olympic dream and be part of the greatest sporting event in the world.
"I actually think it's the greatest moment in British sport and, not only will we have a wonderful seven years leading up to the Games, but for many many years beyond that.
"It'll change the face of sport in this country - right across the UK"
The main Olympic site will be based in Stratford, east London, with the majority of venues in and around the area still to be built.
They include a £255m, 80,000-seater Olympic stadium, a £70m aquatics centre and a £600m athletes' village.
Mayor of London Ken Livingstone warned that any of the businesses on the stadium site which did not accept the relocation terms on offer would face compulsory purchase orders.
But he insisted the regeneration of east London offered a chance to "transform the chances of the children of the East End to break the cycle of poverty in our time".
The Mayor also said the project would not be hijacked by politicians now that the Games had been won.
"One of the reasons we won was that it was so clear our bid, more than any others, was not led by politicians, either at national or city level.
"The Games will be best if they continue to be led by athletes having an input at all levels, and therefore Seb [Coe] and virtually the entire 2012 team will carry on.
"There is no question of stories about us not being ready and up and running," he said.