London police say bombs were placed at three subway stations and on a bus, and at least one person has been injured.
London Police Commissioner Ian Blair says the incidents are "very serious", though not as bad as the July 7 attacks that killed 56 people and wounded hundreds more.
"The casualty numbers appear to be very low in the explosions," he said. "The bombs appear to be smaller than on the last occasion, but we do not know the implications of all this yet and we are going to have to examine the scene very carefully."
Commissioner Blair says the subway system was being shut down as authorities investigate the incidents. He has appealed for Londoners to stay away from the public transport network until further notice.
Witnesses have described a chaotic scene, and several subway riders told reporters they smelled burning insulation or rubber.
Amid the confusion, Prime Minister Tony Blair convened a meeting of his emergency response team, which last met following the July 7 blasts that authorities have blamed on four British Muslim men.