US reaction: The Bush administration raised the terror alert code to orange for mass transit, immediately following the London explosions.
Authorities around the country stepped up security on public transportation systems and city centres and urged the public to be extra vigilant.
"We are asking people to be alert, to report any suspicious activity or packages, backpacks, boxes to police," New York governor George Pataki said.
Homeland security secretary Michael Chertoff decided to raise the threat level to orange, or high, for mass transit, after an emergency meeting with officials and a conference call with US president George Bush in Scotland.
Extra police with automatic weapons and bomb-sniffing dogs were assigned to subway systems and air, rail and bus terminals in Washington, Boston, New York, Miami and Chicago.
In an atmosphere of heightened tension, some cities took independent measures. In San Francisco, rail transport officials locked all station washrooms, and in Boston, Massachusetts governor Mitt Romney ordered his own raised security alert on the city's transport system.
"The governor is alarmed by what happened in London," his spokesman said.
Elsewhere in the US the terror code remains at yellow, the lowest on the colour code to indicate risk of attack.
A police helicopter hovered over Penn Station in Newark, and the taxi lane outside the station was blocked off.
While there was no evidence that the US was being targeted at the same time as London, the nationwide alert was raised for transportation because of vulnerabilities in the system, US counter-terrorism officials said.
"We do not have any specific intelligence indicating this type of attack is planned in the US," said Mr Chertoff, facing his first security challenge since taking over from former homeland security chief Tom Ridge.
The terror alert was last raised in August 2004 when it applied only to the financial districts of New York and New Jersey. National security council and homeland security officials conferred with Mr Bush by secure conference video shortly after the attacks.
Meanwhile, the US State Department told all US embassies around the world to review security arrangements and US secretary of state Condoleezza Rice went to the British embassy to sign a book of condolences and rang British foreign secretary Jack Straw to offer assistance.
The attacks, which US officials are taking to be the work of al-Qaeda, are particularly worrying for the US administration.
Mr Bush has said frequently that the war in Iraq was justified because they had to fight terrorists there so they would not attack the US.
The co-ordinated assault on London transport shows al-Qaeda is still capable of deadly attacks.
One official, however, was quoted as saying that London was considered a prime target in part because al-Qaeda was having difficulty getting people into the US.
Members of Congress condemned the bombings. Senate majority leader Bill Frist who is in Africa, denounced them as "cowardly acts against innocent people" and said: "We stand by the British people in their hour of need as they have done for us." Senator John McCaine said those responsible should be pursued "where they breed".