"We are asking people to be alert, to report any suspicious activity or packages, backpacks, boxes to police," New York Gov. George Pataki said in a telephone interview.

About 4.5 million passengers use New York City subways daily, and officials increased police coverage of the subways, buses and transit stations throughout the city.

Commuters, while admitting to some nervousness, didn't let news of the explosions alter their travel.

"I've always been aware that the subway could be a target but it hasn't affected the way I live my life. I got on this morning and rode downtown just like I always do," said Mary Ellen Kelly, who lives in midtown Manhattan.

Amtrak officials increased security at all its stations around the country, and posted police officers on board all trains.

"These security measures are just taken as a precaution," company spokeswoman Tracy Connell said. "We will continue the heightened security threat level until we have a better understanding of what happened in London."

In San Francisco, Bay Area Rapid Transit System officials closed all station bathrooms, while Massachusetts Gov. Mitt Romney took the unprecedented step of raising the security level on Boston's transit system.

"The governor is alarmed by what happened in London," Romney spokesman Eric Fehrnstrom said. There was no specific intelligence about any threat to the city or the Massachusetts Bay Transportation Authority system, he said.

"You kind of hold your breath until rush hour is over because of the timing of the one in London," said Paul Dullea, 37, of Millis, Mass., while an announcement telling people to report suspicious activity played repeatedly over a Boston subway station intercom.

In Atlanta, all MARTA commuter rail police officers and dogs were deployed. Los Angeles police officers were told they couldn't leave their shifts without permission and extra sheriff's deputies were sent to patrol Metrolink commuter stations and trains, officials said.

A police helicopter hovered over Penn Station in Newark, N.J., this morning while authorities blocked off the lane in front of the station that taxis and buses use. Uniformed officers scurried to each platform and in and out of shops inside the station.

"I'm shaken up," said Towyonia Johnson of Jersey City, who noticed more officers than usual riding the PATH train to her computer operator job in Newark. "If it can happen there, it can happen here. I just want to make it home safe."

Associated Press writers Justin Glanville in New York, Theo Emery in Boston and Wayne Parry in Newark, N.J., contributed to this report.