Two arrests have been made in connection with Thursday's attacks in London, police said.
One man was arrested near Downing Street, the site of Prime Minister Tony Blair's residence, and the other near Tottenham Court Road, which is close to the Warren Street Underground station where one of the incidents took place. Police said the men were being questioned.
In a chilling but far less bloody replay of the suicide bombings that killed 56 people two weeks ago, small explosions struck the London Underground and a bus at midday Thursday.
Only one person was reported injured in the nearly simultaneous lunch-hour blasts, which shocked and disrupted the capital and were hauntingly similar to the July 7 bombings by four attackers.
As CBS News Correspondent Richard Roth reports, the explosions were small, suggesting they may have simply been detonators which failed to trigger deadly bombs, but police said the bombers' objective was plain.
"Clearly, the intention must have been to kill," Blair told a news conference. "You don't do this with any other intention."
He also said it was not clear if the two sets of attacks were connected.
Panicked and screaming commuters fled the three affected Underground stations, sometimes leaving behind their shoes. Firefighters and police with bomb-sniffing dogs sealed off nearby city blocks and evacuated rows of restaurants, pubs and offices.
"We can't minimize incidents such as this," he said at a joint news conference with the Australian prime minister at No. 10 Downing St. "They're done to scare people, to frighten them and make them worried."
He held an emergency Cabinet meeting but said no policy decisions were made.
President Bush was briefed on the explosions and said the terrorists "understand when they kill in cold blood it ends up on our TV screens and they're trying to shake our will. And they're trying to create vacuums in which their ideology can move."
U.S. mass transit systems remain on code orange, or high alert, since the London bombings two weeks ago, but the rest of the country is at yellow, signifying an elevated risk.
New York City Mayor Michael Bloomberg said police will begin conducting random searches of packages and backpacks carried by people entering the city's subway, which carries about 4.5 million passengers on the average weekday. Officials would not immediately say how frequently the checks would occur.
London Transport spokesman Steve Taylor told The Associated Press that it would be impracticable to check bags, or to install airport-style metal detectors and X-ray machines in a subway network that carries 3 million passengers a day, or a bus system that carries some 6 million daily.
Ian Blair called the blasts "a very serious incident."
"We know that we have four explosions or attempts of explosions, and it is still pretty unclear as to what has happened," he said outside Scotland Yard.
"At the moment the casualty numbers appear to be very low ... the bombs appear to be smaller" than those detonated July 7, he said. He added later that not all the bombs went off.
An armed police unit entered University College hospital shortly after an injured person was carried in, Britain's Press Association reported.
Sky News TV reported that police were searching for a man with a blue shirt with wires protruding. Officers asked employees to look for a black or Asian male about 6-foot-2.
The attacks, which targeted trains near the Warren Street, Oval and Shepherd's Bush stations, did not shut down the subway system, only three of its lines. The double-decker bus had its windows blown out on Hackney Road in east London.
Witnesses told The Associated Press they did not hear a bang but smelled something similar to an electrical fire at the Warren Street station.
Witness Jimmy Conners, who was aboard the train at Warren Street, told CBS News there was "the smell of burning wire… we thought we were all going to die… everyone was just panicking and heading for the doors."
Conners said it was "definitely the smell of death. Everyone on that train thought they were going to die."
Roth reports another eyewitness described hearing "explosions that sounded like champagne corks popping."
Police in chemical protection suits were at the Warren Street station, but no chemical agents were found.
Stagecoach, the company which operates the stricken bus, said the driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and there were no injuries.
The incidents paralleled the July 7 blasts, which involved explosions at three Underground stations simultaneously starting at 8:51 a.m., followed quickly by a bomb going off on a bus. Those bombings, during the morning rush hour, also occurred in the center of London, hitting the Underground from various directions.
"This would be the first time we've seen follow-up attacks following any major attack, anyplace in the world,"
Livingstone said the possible implications of the new incidents are frightening. "This sends a very powerful message, and what they're saying is, 'No matter how great your network of informants and cameras and so on, you still can't stop us. You can arrest us. We can have people that are killed in these attacks, but we have other people that are willing to carry out further attacks. And so we're going to be there, and we're going to be there for a long time.'"
Thursday's strikes were more spread out than the July 7 blasts. Emergency teams were sent to all three stations after the incidents, which began at 12:38 p.m.
"People were panicking. But very fortunately the train was only 15 seconds from the station," witness Ivan McCracken told Sky news.
McCracken said another passenger at Warren Street told him he saw a backpack explode. The July 7 bombs were carried in backpacks, police said.
McCracken said he smelled smoke, and people were panicking and coming into his carriage. He said he spoke to an Italian man who was comforting a woman after the evacuation.
"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack," McCracken said. "The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."
Losiane Mohellavi, 35, who was evacuated at Warren Street, said, "I was in the carriage and we smelled smoke — it was like something was burning. Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking."
The U.S. Embassy was closed to visitors about two hours after the blasts as a precaution, but embassy staff continued working, said spokeswoman Susan Domowitz.
The explosions came as Pakistani intelligence officials said authorities are seeking the former aide of a radical cleric in Britain in connection with the July 7 bombings.
The officials said British investigators asked Pakistani authorities to search for Haroon Rashid Aswat, who reportedly had been in close contact with the suicide bombers just before the July 7 attacks.
Aswat, 31, was of Indian origin and may not be in Pakistan, according to two intelligence officials in Islamabad and one in Lahore, all speaking on condition of anonymity because they are not authorized to talk to the media and because of the sensitivity of the investigation.
Aswat reportedly was once an associate of Abu Hamza al-Masri, a radical imam awaiting trial in Britain on charges of incitement to murder. Al-Masri also is wanted in the United States on charges of trying to establish a terrorist training camp in Bly, Ore.; involvement in hostage-taking in Yemen; and funding terror training in Afghanistan.
Quoting unidentified intelligence sources, The Times of London said Aswat visited the hometowns of the four London bombers and selected their targets. It also reported there had been up to 20 phone calls between Aswat and two of the bombers before the attacks.
Aswat's relatives in Batley, near the northern English town of Leeds, which was home to two of the July 7 suicide bombers, said they had not heard from him for many years.
"He has not lived at this house and we have not had contact with him for many years," said his father, Rashid, who asked for his family to be left in peace. "There is no story that we can provide."
Authorities are investigating whether the London bombing suspects, three of whom were of Pakistani origin and traveled to Pakistan last year, received training or other assistance from militants in that country.
One of the July 7 bombers, Shahzad Tanweer, 22, is suspected of visiting a madrassa linked with militants in Lahore which has become a focus of the inquiry.
According to a report in a Pakistani newspaper, Tanweer revered Osama bin Laden. The English-language Dawn newspaper said Tanweer visited relatives in November in a farming village near Faisalabad in eastern Pakistan. During his stay, he was visited by another bombing suspect, Mohammed Sidique Khan, 30, Tanweer's uncle told the newspaper.
Pakistan has pledged to curb religious extremism amid international concerns that Islamic schools, or madrassas, are promoting extremism.