Within the next hour, Oval and two other stations were evacuated, and London Transport shut down three of its subway lines.
"We are responding to reports of incidents at Warren Street and Oval underground stations," James Buckley, a spokesman for London Transport announced. "There are no reports of any casualties at this stage. We have suspended three tube [subway] lines -- that's Hammersmith and City, Victoria, and Northern."
It has been two weeks since suicide bombers struck London's transport system at the height of the morning rush hour.
More than 50 people died in those attacks.
Today, the man leading that enquiry confirmed that, again, a series of explosions had hit London's transport system.
"We know that we have four explosions or attempts at explosions, and it is still pretty unclear as to what has happened," said London Police Commissioner Ian Blair. "At the moment, the casualty numbers appear to be very low in the explosions. The bombs appear to be smaller than on the last occasion, but we don't know the implications of all this yet and we are going to have to examine the scene very carefully."
Train passengers who had fled reported panic and chaos below ground.
As news of the explosions spread, Prime Minister Tony Blair and London's mayor postponed their engagements.
Events were still unfolding in the late afternoon, however.
Officers in chemical protection suits have been sent to two of the evacuated underground stations, though they've found no trace of chemical agents at one of them.
Armed police were also seen entering a hospital near the Warren Street underground station.