A rush-hour traveller was pushed to his death in front of a Tube train in an apparently random attack, police said.
The man, in his 50s, was standing on a platform as the train pulled into Highbury and Islington Station in north London on Friday.
Police, who are treating his death as murder, arrested a man early this morning and are appealing for witnesses to the incident on the Victoria Line.
It is not believed the victim knew the man who pushed him.
Chief Superintendent Paul Crowther, of British Transport Police said: "This is a very early stage of our investigation.
"It was 6pm in the middle of the rush hour and this was a very busy, packed platform.
"We need to piece together exactly what happened in the run up to this incident.
"We are unclear at the moment about the exact circumstances. There doesn't appear to have been any incident running up to it."
Two years ago 2004 drug addict Stephen Soans-Wade, of Poplar, east London, was jailed for life for pushing security guard Christophe Duclos to his death in front of a rush-hour Tube train.
Soans-Wade became violent after being turned away from hospital and shoved Frenchman Mr Duclos, 37, off the platform at Mile End, east London, on September 13, 2002.
He had claimed he was guilty of manslaughter on the grounds of diminished responsibility, claiming voices had told him to do it, but the jury convicted him of murder.
The Victoria Line was closed between Warren Street and Walthamstow Central following Friday's incident.
It did not reopen on Saturday because of planned engineering works to re-lay track.
A spokesman for Transport for London said this section of the Victoria Line was expected to open again as usual on Monday morning.