The most powerful men in sport held a moment's silence at the start of the International Olympic Committee's Session on Friday in tribute to those killed in deadly bomb attacks on London [Images].

"We begin with a painful issue," IOC president Jacques Rogge said a day after four blasts tore through London's transport system, killing dozens of people in a coordinated rush-hour attack.

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"The IOC was appalled by the barbaric attack on the heart of London. I have sent, on your behalf, condolences to Tony Blair [Images], Ken Livingstone and Sebastian Coe.

"This tragic act has sadly proved that there is no safe haven on this earth and that security must remain the first priority wherever Olympic Games are staged."

Thursday's deadly attack -- the capital's deadliest in peacetime -- came one day after London had been awarded the 2012 Olympic Games.

IOC member Craig Reedie said they would not affect the city's determination to stage successful Games.

"I can assure you that these terrible acts in no way reduce our resolve to run excellent Games in 2012," Reedie told fellow members. "The worst thing would be to concede to these people."

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Reedie said the events of Thursday wiped out all the "pride, pleasure and euphoria" London 2012 had felt at overhauling long-time favourite Paris to win the final round of voting 54-50 to win the Games.

Australian Prime Minister John Howard, a close ally, said on Friday he had been advised the death toll from the four explosions stood at 52. Police in London had said on Thursday 37 people died in the attacks and that about 700 others were wounded.

Prime Minister Tony Blair rushed to London from a meeting of the leaders of the Group of Eight rich nations in Scotland after branding the attacks "barbaric".

He returned to the summit later, pledging not to allow the attacks to thwart moves to fight African poverty and climate change.

A previously unknown group, "Secret Group of al Qaeda's Jihad in Europe", claimed responsibility for Thursday's attack.

Britain is the closest ally of the United States in Iraq, where al Qaeda is waging a bloody insurgency.