UPDATED: Dog-on-a-string brigade attack G20
The man fell over unconscious, prompting a member of the public to call emergency services.
Police "made extensive efforts to resuscitate him both there and on the way to hospital'', an emergency services spokesperson said.
The man's death came as police, many dressed in riot gear, were out in force as thousands demonstrated through the city's financial district, outside the US embassy and in Trafalgar Square.
Chanting anti-capitalist, anti-war and anti-pollution slogans, they vented their fury as G20 leaders prepare to discuss the world financial crisis tomorrow.
Read it here.
UPDATE: It turns out that the police who tried to administer first aid to this person, who was a passer-by, were themselves pelted with missiles - disgusting. The UK Daily Mail has the full story.