Witnesses travelling on the London Underground have spoken of the panic as three Tube stations were evacuated following separate incidents shortly before 1pm.
A British Transport Police spokeswoman said Warren Street, Shepherds Bush and Oval stations had all been evacuated. She said the incidents were "ongoing". Underground services were suspended as the alert spread.
Sosiane Mohellavi (35) was travelling from Oxford Circus to Walthamstow when she was evacuated from a train at Warren Street.
"I was sitting in the carriage reading a book and I smelt something burning, like wiring or tyres, and it just got more intense. Suddenly people panicked and started screaming and were walking on each other's backs trying to get the hell out of there.
"I couldn't move, I didn't know what to do, whether to run or not. People ran and left their shoes and belongings when they smelt the burning," Mr Mohellavi said.
Victoria Line passenger Ivan McCracken claimed a traveller's rucksack had exploded on the Tube outside Warren Street station.
He told Sky News: "I was in a middle carriage and the train was not far short of Warren Street station when suddenly the door between my carriage and the next one burst open and dozens of people started rushing through. Some were falling, there was mass panic.
"It was difficult to get the story from any of them what had happened but when I got to ground level there was an Italian young man . . . 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.
"The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."
Mr McCracken said he smelled smoke but did not see any injured passengers.
Stagecoach, which owns the number 26 bus involved in one of the incidents, said the driver heard a bang at around 1.30pm. The bus had left Waterloo and was in Shoreditch when the incident happened."The driver heard a bang which appeared to come from the upper deck. When he went upstairs to investigate, the windows on the upper deck were blown out."