LONDON - A second wave of bombings echoed through London's transit system yesterday, shutting down much of the city for hours and leaving commuters and residents jittery with fears that a sustained campaign of violence may be under way.

One person was reported injured in what police described as "an attempt" by bombers to explode bombs on three subway trains and a double-decker bus, the same number and types of targets that were struck by suicide bombers July 7 in attacks that killed 56 people and injured more than 700.

Police said last night that they arrested two people in the vicinity of the trains that were targeted, but they released one person without charges and privately cautioned that there was no evidence connecting the other man to yesterday's incidents.

"Clearly, the intention must have been to kill," Ian Blair, the police commissioner who oversees Scotland Yard, said of yesterday's attacks. "You don't do this with any other intention."

Yesterday, as two weeks earlier, thousands of commuters fled the Underground trains in terror, women leaving behind shoes, men leaving behind briefcases as they sought safety above.

Only this time, no mangled trains or mangled bodies were left behind on the subway system, known to Londoners as the "Tube."

Three million passengers ride the subway each weekday; 6 million take London's vast network of buses.

Caroline Russell, 42, said she and other passengers smelled what seemed to be burning rubber deep underground on the Victoria Line. When the train stopped at the Warren Street station, in North-Central London, she said, a rush of panicked passengers from another car on the train began running for the exits.

"There was just a stampede, with people running, crying, really upset," she said. "We all moved to a parallel tunnel, and I knew we had to get out of there, and then this Italian girl who was all by herself collapsed at my feet."

Speaking to another passenger once she helped the Italian girl to street level, Russell said, she learned from a passenger on another car that a backpack being carried by a young man had "exploded but not real loudly." Passengers from that car pulled the emergency cord and then ran through connecting doors into another car while the train was moving toward Warren Street.

"We were all shaken," she said. "But we all felt fortunate."

Police were releasing few details about the attacks, but witnesses reported hearing relatively low-level explosions beginning about 12:25 p.m., and London's subway system was evacuated.

Not all of the bombs exploded, the commissioner said, and he left unclear whether only detonation devices may have gone off or whether bombs that may have exploded were simply not powerful enough to do serious damage. Last night, bomb squads were on hand in Hackney, where an explosion on the No. 26 bus blew out several windows.

The BBC, citing intelligence officers, reported that the recovered explosives were similar in makeup to those used in the July 7 attacks.

A shaken city

Police with high-powered weapons were on streets where normally only unarmed bobbies are seen, emblematic of how London, despite residents' efforts to get on with their lives, remains a city shaken by two series of attacks in as many weeks.

The London Eye, the giant Ferris-wheel-like attraction on the River Thames, was emptied of its passengers and closed. The U.S. Embassy turned away afternoon appointments.

Large expanses of London's streets, usually with bumper-to-bumper traffic during the noon hour, were closed to traffic, which diverted into miles-long traffic jams elsewhere, blue and white barrier tape snapping in the wind.

Office buildings were evacuated. Pubs were emptied. People stood on sidewalks and in the closed streets asking one another for information and seeking ways to get home.