A spokesman for the Royal Sussex County Hospital said the man died at 2.30pm.
England cricketer Matt Prior had officially started today's race, the fourth Brighton marathon, in Preston Park at 9am.
Organisers said 9,315 competitors were taking part.
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The family of a Sussex teenager who died after just one punch during a night out has reached a milestone in their lifesaving campaign.
The headteacher of a Berkshire secondary school has quit over funding cuts which she says means children will suffer.
Troops from the 4th Battalion the Rifles returned home today after spending four months on deployment in Iraq.