Stations at Warren Street, Oval and Shepherd's Bush have been closed.
British Transport Police say one person has been injured at Warren Street station.
Services on the Victoria, Northern and Hammersmith & City Lines have been suspended.
Emergency services started receiving calls just after 12.30.
At Warren Street there were reports of shots and a nail bomb explosion. Sky's Crime Correspondent Martin Brunt said police believe this may have been the sound of detonators going off.
A Transport for London spokesman said: "There is a Code Amber which means that the trains are being taken to the next station and passengers evacuated to above ground.
"Police are at all three stations."
Victoria Line train passenger Ivan McCracken told Sky News he spoke to an Italian man who witnessed an explosion just after the train arrived at the platform.
"He told me he had seen a man carrying a rucksack which suddenly exploded. It was a minor explosion but enough to blow open his rucksack. Everyone rushed from the carriage. People evacuated very quickly. There was no panic.
"I didn't see anyone injured but there was shock and fright.
"There was a smell of smoke."
"The man who was holding the rucksack looked extremely dismayed."
One witness told Sky News that passengers tried to prevent a man with a rucksack running away but they failed.
Police have cordoned off the streets around Warren Street station. Sky reporter Mark White said authorities are "pushing everybody as far back from the station as they can".
"Police activity suggests that they are looking for somebody on the outside here."
Another eyewitness, Sosiane Mohellavi, said "Everyone panicked and people were screaming. We had to pull the alarm. I am still shaking."
Scotland Yard said emergency services have responded to an "incident" on a Number 26 bus in Hackney Road, on a junction near Colombia Road, east London.
Sky News Foreign Affairs editor Tim Marshall said: "There is a bus parked there which is empty and there is nobody anywhere near it. This does indicate that police think the bus is still dangerous."
Sky News Online's Rob Cole is at Shepherd's Bush. He said: "The whole of Shepherd's Bush Green is sealed off. About 400 metres on both sides of the Tube station is affected."
Network Rail say all mainline train services are running from London stations.
It is two weeks to the day since bombers attacked three Tube trains and a bus in central London.
interesting...all i keep hearing is "witnesses saw a *man*"
well what kind of "man" was he....MUSLIM????
A gun might have helped, with the shooter careful to avoid injuring innocent people of course.
Great summary, thanks.