The alert "targeted only to the mass transit portion of the transportation sector," said Homeland Security Secretary Michael Chertoff. "This includes regional and inner city passenger rail, subways, and metropolitan bus systems."
Airports and air travel were not included in the alert.
The terror alert had not been raised in the United States since last August, when the Homeland Security Department increased it to orange — or high — for financial institutions in Washington, New York and Newark, N.J., in the run up to the November elections.
"We are not suggesting that people avoid public transportation systems," Chertoff added, only that riders keep an eye out for problems.
President Bush, in Scotland for the G-8 summit,.
"Currently, the United States has no specific, credible information suggesting an imminent attack here in the United States," Chertoff said.
"I actually think we have a very safe system, but the fact remains we've had an incident in London," he said. "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."
President Bush had a 10-minute video conference call with National Security advisers back in the U.S., reports CBS News White House Correspondent Mark Knoller.
In a brief speech,
"The war on terrorism goes on," he said. "We will not yield to these people. We will not yield to the terrorists."
Terrorism analyst Neil Livingstone tells CBS radio affiliate WTOP in Washington that "it's not a question of if, but when" the U.S. will be attacked again by al Qaeda.
"This is a war that's going to have peaks and valleys," Livingstone said. "Since 9/11, nothing has happened. A lot of Americans think that Osama bin Laden's on the run, therefore we have nothing more to fear."
At the Pentagon, police officers said they had been placed on a heightened alert shortly after the London attacks.
The State Department told all U.S. embassies to review their security arrangements. Secretary of State Condoleezza Rice telephoned British Foreign Secretary Jack Straw offering any assistance his government might require, said a State Department official, who spoke on condition of anonymity because the official was not authorized to be identified.
The U.S. Embassy in London is secure, but the consular section, which deals with visas and other routine business, is closed, the official said.
Security around the massive home of the Defense Department, hit in the Sept. 11 attacks, was more visible than normal Thursday, with police officers in squad cars, on bicycles and on foot deployed in greater numbers than usual on the perimeter roadways and the Metro station.