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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Owners must also prove they have something to pick up dog mess with under new rules imposed by one council.
Monday's weather for the east
Monday's weather for the west