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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A man accused of murdering his father says he lost control after a lifetime of being bullied by him.
Tuesday night's weather for the east of the region
Tuesday night's weather for the west of the region