Two weeks after bomb attacks brought London to a halt, three Underground stations have been evacuated and three lines closed after a series of explosions.
Police said the emergency services had also responded to reports of a blast on a bus in the Hackney area. One person was reported injured in the incidents.
A spokeswoman said they were not treating the reports of emergencies in the central London transport system as a "major incident" yet.
But London police chief Ian Blair said the incidents were "clearly very serious" although the blasts appeared to be smaller than those which occurred in the capital two weeks ago. He said there were four explosions or attempted explosions.
The blasts were said to have been triggered by detonators containing no explosives.
Police were called out to three central London stations: Warren Street, Oval and Shepherd’s Bush. Large areas around the stations were cordoned off.
One person was injured at Warren Street. Their condition was unknown.
The whole of the Northern Line was suspended, along with the Victoria Line and the Hammersmith and City line.
The three stations were evacuated after the fire brigade received reports of smoke coming out of a train.
"I was in the carriage and we smelt smoke – it was like something was burning," said a passenger evacuated from a train at Warren Street.
Early reports also spoke of a nail bomb explosion.
In the east of the city an explosion blew out the windows of a bus. The driver reported no injuries.
The incidents were reported at lunchtime on Thursday, exactly two weeks after four explosions in the London transport system, which killed 56 people.
swissinfo with agencies
On Thursday June 7, three bombs exploded on or close to the London Underground and a bus was blown up.
The terrorist attacks caused 56 deaths and injured around 700.