A British national died in Sofia hours before England's Euro 2020 qualifier, Bulgaria's Ministry of Interior has confirmed.
Police were called to help the 32-year-old, thought to be an England supporter, at 10am local time (8am UK time).
In a statement, the authorities said the man was in a "helpless condition" and "suddenly began to act aggressively, raging and threatening".
A spokeswoman said: "A 32-year-old man has sadly died.
"Police are working to clarify all the circumstances in this incident."
A cause of death has yet to be identified.
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In a separate incident, an injured fan was taken away in an ambulance after a clash at a bar in Sofia
Police said one person was treated for hand injuries and another was arrested. Both were foreign nationals.
It came as England won their match against Bulgaria 6-0, but the UK's minister for sport Nigel Adams has said he expects "tough action" from Uefa after the game was twice halted due to racist abuse from the stands.
Tonight the @England team have been subjected to disgusting abuse. Racism should never be tolerated. Discrimination must be stamped out from the game and I expect tough action from @UEFA to follow. The @England players have my full support. 🏴
— Nigel Adams MP (@nadams) October 14, 2019
He tweeted: "Tonight the @England team have been subjected to disgusting abuse. Racism should never be tolerated.
"Discrimination must be stamped out from the game and I expect tough action from @UEFA to follow. The @England players have my full support."
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The vile racism from some Bulgarian fans tonight shames their country and their team.@England outclassed them in every way, but no player should be subjected to that treatment.
— Sadiq Khan (@SadiqKhan) October 14, 2019
Enough is enough. There should be severe consequences from @UEFA for any racist abuse. #BULENG
Mayor of London Sadiq Khan tweeted: "The vile racism from some Bulgarian fans tonight shames their country and their team."
Mr Khan continued: "@England outclassed them in every way, but no player should be subjected to that treatment.
"Enough is enough. There should be severe consequences from @UEFA for any racist abuse."
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