WASHINGTON – In the wake of yesterday’s deadly bombings in London, the federal government triggered tighter security for subway, rail and bus systems but left the nation’s overall alert level unchanged.

The secretary of homeland security, Michael Chertoff announced at a midday press conference that the “threat advisory” for mass transit was being raised to orange, or high, from yellow, or elevated. “We feel at least in the short term we should raise the level here because, obviously, we’re concerned about the possibility of a copycat attack,” Mr. Chertoff said.

The homeland-security chief said the change was dictated by “common sense” and not by any specific indication that an attack on American commuters was imminent. However, Mr. Chertoff acknowledged that there was also no intelligence warning of the London attacks.

“This is not an occasion for undue anxiety,” he said. “I actually think we have a very safe system.”

As the first bomb went off in London, President Bush was meeting with British Prime Minister Blair in Scotland during the G-8 Summit. The president was informed of the attacks about 20 minutes after they were first reported, the White House press secretary, Scott McLellan said.

Mr. Bush later used a secure video link to convene a meeting with his national security team.

In a statement to reporters, the president offered condolences to the residents of London and praised Mr. Blair’s leadership. “The war on terror goes on,” Mr. Bush said. “We will not yield to these people, will not yield to the terrorists.”

Mr. Bush also said he was struck by the dichotomy between the death and destruction unleashed in England and the efforts he and the other leaders at the summit were taking to alleviate poverty and AIDS. “The contrast couldn’t be clearer between the intention and the hearts of those of us who care deeply about human rights and human liberty, and those who kill – those who have got such evil in their heart that they will take the lives of innocent folks.”

In Washington, Secretary of State Rice visited the British Embassy to sign a condolence book. A spokesman said American embassies worldwide were reviewing their security precautions.

A flurry of statements deploring the attacks emerged via e-mail from congressional offices. Senator Schumer expressed condolences to the victims and said the bombings highlighted the need for sharp increases in security funding for American mass transit. “Following the attack on Madrid’s rail system, the terrible terrorist attacks in London is our second wake-up call,” Mr. Schumer said. “The soft underbelly of buses and subways and railroads are fully exposed to similar terrorist attacks unless we take real steps to beef up mass-transit security immediately.”

Mr. Schumer said he wants to double mass-transit and rail security funding in a pending homeland security appropriations bill from $100 million to $200 million. He also wants to double spending for bus security improvements from $10 million to $20 million.

Asked about the proposal, Mr. Chertoff said his department was preparing to unveil a new set of priorities that will address infrastructure protection. “I wouldn’t make a policy decision driven by a single event,” he said.

Senator Clinton was returning to America from Singapore as the attacks took place yesterday. “This brutal and senseless act is a tragic reminder that we need to do everything we can to safeguard our own transit systems,” she said in a statement that also expressed condolences to those directly affected by yesterday’s bombings.

Near the Capitol, some transit and tour buses were being stopped and inspected by police, said the chief of Washington’s transit police, Polly Hansen.

There were few indications of stepped-up security during the morning rush hour, but by evening police appeared to be stationed at every subway entrance. Transit employees donned high-visibility orange vests and handed out cards to passengers urging them to be on the lookout for suspicious packages or individuals. Some officers carried automatic weapons on patrols, while others led bomb-sniffing dogs, police said.