Mr. Bush said he spoke with federal homeland security officials back in Washington.
"I instructed them to be in touch with local and state officials about the facts of what took place here and in London and to be extra vigilant as our folks start heading to work," the president told reporters from a summit of world leaders here.
"The war on terror goes on," he said. "We will not yield to these people, we will not yield to the terrorists."
In Washington, the Homeland Security Department asked authorities in major cities for heightened vigilance of transportation systems, though the nation's threat level was not raised.
The president offered the "heartfelt condolences" of the American people to the victims and their families in London.
Minutes before, British Prime Minister Tony Blair had read a joint statement from all the leaders gathered at the summit being held at a posh resort here.
President Bush said he and the other leaders wanted to send a message of solidarity, reports CBS News White House Correspondent Mark Knoller.
"I was most impressed by the resolve of all the leaders in the room," Mr. Bush said. "Their resolve is as strong as my resolve."
Blair then rushed to London.
A half-dozen 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. Deaths and injuries mounted and officials shut down the entire underground transport network.
Security at the summit was not affected, and Mr. Bush had no plans to return to Washington early. But G-8 leaders took a long break in their morning opening session so they could get individual briefings on developments.
White House spokesman Scott McClellan said the president received frequent updates from Chief of Staff Andrew Card and National Security Adviser Stephen Hadley. Card also notified Vice President Dick Cheney, McClellan said.
Officials have been monitoring events in London from the White House Situation Room, reports CBS News Correspondent Peter Maer.