The 27-year-old man from Hornsey, north London, was with his brother on the northbound Piccadilly Line platform at Finsbury Park Underground station when a man approached from behind, a Scotland Yard spokesman said.
'The man didn't speak but stood so close that they began to move away,' a spokesman said.
'At this point he put his hand in front of the victim and stabbed him in the face with a knife. The attack appears motiveless.'
The victim, who has not been named by police, died in hospital.
A man was last night being questioned by police.Reuse content