"We know why these things are done. They are done to scare people and to frighten them, to make them anxious and worried," said the prime minister. "Fortunately, in this instance there appear to have been no casualties. The police have one their very best, and the security services too, in the situation. And I think we've just got to react calmly."
British police said the Warren Street, Shepherds Bush, and Oval underground stations had been closed. At least three underground lines have reportedly been closed by authorities: Northern, Victoria and Hammersmith/City.
Many of the metro passengers were reported close to panic as they evacuated the stations.
Prime Minister Blair had already planned a meeting today with intelligence and police representatives to talk about the possibility of extending their powers following the 7 July bombings in London.
There were reports of minor explosions on trains on these lines, possibly caused by detonators, but no major bomb blasts. There was also a minor explosion on a bus in East London, which blew out the bus windows.
London police chief Ian Blair said that in all there were four explosions or attempted explosions.
"We know that we have four explosions or attempts at explosions and it is still pretty unclear as to what has happened," said police chief Blair.
He said there were few casualties, but no details were immediately available.
Men in chemical-protection suits were seen entering one of the subway lines shortly after the incidents began, but early reports said no chemical agents had been found.
"U.K.: New London Explosions Follow Deadly Attacks By Two Weeks"
"Pakistani Authorities Question Suspects In London Probe"
"Blair Seeks Alliance With Muslims Against Extremism"
"Analysis: Can Subways Be Protected?"