Heavily armed SWAT teams put on a show of force in New York's subway and Coast Guard boats escorted the Staten Island Ferry across the harbor.
Surveillance cameras watched the rails in Boston.
Extra sheriff's deputies patrolled Los Angeles Metrolink trains.
And from San Francisco to Washington, mass riders were being warned to stay alert and report suspicious activity.
"We really need people to trust their intuition and their observation skills about what doesn't look right," said Chief Polly Hanson of Washington's Metro system.
Her transit cops began carrying machine guns Thursday.
In raising the terror threat level for mass transit, officials concede there's no surefire way to defend buses and trains from terror attacks, reports CBS News Correspondent Bob Orr.
About 29 million people in the United States take commuter trains or subways on an average workday, and millions more take buses. The New York City area accounts for about a third of the rail total, followed by Chicago, Washington, Boston and Philadelphia. The West Coast's largest transit system is in San Francisco.
Metal detectors and explosive-sniffers are not practical, and there aren't enough police officers to patrol thousands of rail stations.
"The government can't be everywhere at all times, particularly in these wide open transportation systems. What we rely on is a vigilant public, an alert public," said security analyst David Heyman.
Even in an age of high-tech security equipment, local transit authorities in the United States are relying mainly on people and dogs to keep their systems safe from terror attacks.
"Very little technology can be applied in this area in an effective way," said Rafi Ron, a Washington security consultant who formerly headed security at Israel's Ben Gurion Airport.
The rush-hour London bombings prompted the United States to put its subways, buses and commuter trains on high alert, moving up one notch to code orange, the second-highest level on the Homeland Security Department's color-coded terror alert scale, for mass transit. Airplanes were not included.
said it was issuing a bulletin to federal, state, local and private sector officials reminding them that the rail system remains an attractive target of the al Qaeda network.
But Department Secretary Michael Chertoff cautioned against panic.
"I want Americans to know that our transit system is safe," he said Friday on CBS News' The Early Show. "I want them to be vigilant, but I want them to go about their business and continue to pursue their daily life."
Asking riders to pay more attention to their surroundings has two effects.
"It helps us detect if something is going on. It works as a deterrent," Chertoff told co-anchor Harry Smith.
The impact of the London attacks was felt across America.
In Denver, Dave Raven said he had planned to take a bus on Thursday but changed his mind and climbed into his car for the commute from nearby Boulder after hearing about the bombings.