A Wales fan died after an assault was reported outside Wembley Stadium before Tuesday's match between Wales and England in the Euro 2012 qualifying, authorities said.
The London Ambulance Service said the 44-year-old man suffered a heart attack before kickoff outside the north London stadium and police confirmed that he later died in a hospital.
Six men, who police believe to be Wales supporters, were arrested in connection with the incident which is now being investigated by the Metropolitan Police's murder team.
The match went ahead, with England winning 1-0, and details of the incident emerged about two hours after the final whistle.
The emergency services were called to the north London stadium 25 minutes before the 7:45 p.m. kickoff.
Police said in a statement that officers were called "to an area outside Wembley Stadium following reports of an assault."
London Ambulance Service spokesman Alistair Drummond said its staff at the event "attended to the patient within three minutes."
"They treated a 44-year-old man who had suffered a cardiac arrest," Drummond said. "Extensive efforts were made to resuscitate the patient and he was taken to hospital."
The victim has yet to be identified.
"Police believe they know the man's identity, but await formal identification and confirmation that all next of kin have been informed," police said. "A post mortem will be scheduled in due course. Officers from the Homicide and Serious Crime Command are investigating."
Cardiff City confirmed that the victim also supported the Welsh capital club which plays in the second tier of English football.
"Late on Tuesday night, details reached us concerning a tragedy where a Wales and Cardiff City supporter died at the match between England and Wales," the club said in a statement on its website. "Out of respect for the family of the supporter, the sensitivity of the situation and until full details and circumstances are ascertained, no statements will be made until deemed fit by the proper channels."