GLENEAGLES, Scotland – World leaders rallied behind Prime Minister Blair yesterday 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.

Mr. Blair, host of the Group of Eight summit, rushed home to tend to the crisis. He returned hours later, in time for a working dinner and Friday’s concluding session. “We will not allow violence to change our societies or our values nor will we allow it to stop the work of this summit,” Mr. Blair said after the attacks. The flags of all nations at the summit were lowered to half-mast.

President Bush, in a series of secure videoconferences with Washington, directed American homeland and national security officials to take extra precautions. America later raised its terror alert to orange, or high, for the nation’s mass transit systems.

“The war on terror goes on,” Mr. Bush told a hastily assembled group of reporters on the lawn of the Gleneagles Hotel as Mr. Blair’s helicopter lifted off behind him, headed south toward London. Mr. 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 Messrs. Bush and Blair over the war in Iraq, came together in pledging solidarity. Chancellor Schroeder called the bombings “perfidious attacks.” Said President Chirac: “This scorn for human life is something we must fight with ever greater firmness.”

Mr. 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 – America, Britain, Japan, Germany, France, Italy, Canada, and Russia, plus the leaders of China, India, Mexico, Brazil, and South Africa – Mr. Blair said the attacks were “not an attack on one nation but on all nations and on civilized people everywhere.”

The British foreign secretary, Jack Straw – who temporarily replaced Mr. Blair as summit chairman – said the attacks have the “hallmarks of an Al Qaeda-related attack.” Events in London quickly overshadowed debate on the summit’s two major issues: global warming and increased aid to fight poverty in Africa.

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