British Prime Minister Tony Blair (search) said deadly explosions in London were the work of terrorists "designed and aimed to coincide" with a summit of world leaders.

Blair said he was leaving the summit for the day to return to London, while the other leaders continued the meeting.

"Each of the countries around that table has some experience of the effects of terrorism and all the leaders, as they will indicate a little bit later, share our resolution to defeat this terrorism," a shaken Blair said in a brief statement to reporters.

"It's particularly barbaric that this has happened on a day when people are meeting to try to help the problems of poverty in Africa, the long-term problems of climate change and the environment."

This year's summit of the G-8 (search) countries — the United States, Britain, Canada, France, Germany, Italy, Japan and Russia — was being held at an exclusive 850-acre golf resort under the kind of heavy security that has been the norm at these gatherings since the Sept. 11, 2001 terrorist attacks in the United States.

At least six blasts rocked the London subway and tore open at least one packed double-decker bus in nearly simultaneous explosions during Thursday's morning rush hour. At least two people were killed and officials shut down the entire underground transport network.

"Just as it is reasonably clear that this is a terrorist attack or a series of terrorist attacks, it's also reasonably clear that it is designed and aimed to coincide with the opening of the G-8," Blair said.

There was no immediate word on who was responsible.

White House spokesman Scott McClellan said Bush had been briefed, but offered no other details. Secret Service spokesman Tom Mazur said that Bush's presence had agents monitoring the situation in London, but that the investigation was being left to British authorities.

Liz Kirkham, spokeswoman for Tayside Police Force, which covers the Gleneagles area, said no additional security precautions were being taken at the summit as a result of the blasts, as substantial measures had already been put in place.

"Whatever they do it is our determination that they will never succeed in destroying what we hold dear in this country and other civilized nations throughout the world," Blair said.