Bob Geldof asked people to think about the plight of Africa as he spoke at the start of the Gay Pride event.
Gay Pride Day is the culmination of a fortnight of festivities
He addressed the thousands taking part in the annual parade and festival on the streets of London, at Park Lane, central London, on Saturday.
The rally took place in Trafalgar Square and a cabaret in Leicester Square will go on into the evening.
Pride Day is the culmination of a fortnight of events in aid of funds for a lesbian and gay community centre.
Geldof told the marchers: "Between what you people are doing and the people in the park are doing and the people around the world, we are going to stop one vast oppression of a vast minority - that's what we are going to do today."
He also joked that he would take "a chance to go mincing about in London" before U2 opened the Live 8 gig at 1400 BST.
More than 40 floats, marching groups, dancing groups and bands have made their way along Park Lane, through Westminster, past the Houses of Parliament and along Victoria Embankment.
Actor Sir Ian McKellen, mayor Ken Livingstone and gay and human rights campaigner Peter Tatchell were among speakers at the rally.
Tributes are also being paid to gay barman David Morley, who was beaten to death on the South Bank in October 2004.