World leaders rallied behind British Prime Minister Tony Blair (search) on Thursday in resolving to defeat terrorists after a series of deadly subway and bus attacks in London abruptly took over the agenda of the annual meeting of leading nations.

Blair, host of the Group of Eight (search) summit, rushed home to tend to the crisis but pledged to return for Friday's concluding session.

"We will not allow violence to change our societies or our values," he said.

The United States later raised its terror alert to orange, or high, for the nation's mass transit systems.

"The War on Terror goes on," President Bush told a hastily assembled group of reporters on the lawn of the Gleneagles Hotel as Blair's helicopter lifted off behind him, headed south toward London (search).

Bush said the resolve of other summit partners in combating terrorism "is as strong as my resolve. We will not yield to these people, will not yield to the terrorists."

The summit briefly recessed so leaders could gather information on the attacks. Leaders also postponed until Friday planned declarations on climate change and the global economy.

The G-8 leaders, many of whom had differed sharply with Bush and Blair over the war in Iraq, came together in pledging solidarity.

German Chancellor Gerhard Schroeder called the bombings "perfidious attacks." Said French President Jacques Chirac: "This scorn for human life is something we must fight with ever greater firmness."

Blair and other leaders said it was clear the morning rush-hour attacks that killed dozens and wounded hundreds were timed to coincide with the opening business session of the summit.

In a statement on behalf of all 13 participants — the United States, Britain, Japan, Germany, France, Italy, Canada and Russia, plus the leaders of China, India, Mexico, Brazil and South Africa — Blair said the attacks were "not an attack on one nation but on all nations and on civilized people everywhere."

Events in London quickly overshadowed debate on the summit's two major issues: global warming and increased aid to fight poverty in Africa.

Now, Friday's final session seems just as likely to be dominated by discussions of the war on terrorism.

Advocacy groups, however, were hopeful the summit could get back on track.

"The violence in London only underscores the need for a strong G-8 action on Africa," said John Brennan with the Washington-based Bread for the World.

Earlier, Bush and Blair met but could not bridge differences over climate change. Blair had wanted a strong summit statement setting specific targets for reductions in carbon dioxide and other pollutants believed to contribute to global warming. Bush argued for more flexibility, more joint scientific research and for bringing developing nations like China into a new round of talks.

"We're not going to resolve every single issue at the G-8 summit in relation to this," Blair said at a joint news conference with Bush after breakfast.

Shortly after that, reports of the explosions started rolling in.

"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," Blair said later.

White House spokesman Scott McClellan said Bush was briefed in more detail by his chief of staff, Andrew Card, and national security adviser Stephen Hadley, and was updated throughout the day.

Canadian Prime Minister Paul Martin extended condolences to the victims. Russian President Vladimir Putin said through spokesman Alexei Gromov that "no matter where such inhuman crimes occur in London, New York, Moscow or other countries of the world — they demand unconditional condemnation."

Italian Premier Silvio Berlusconi blamed "a band of fanatical criminals" for the attacks.

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