A funding shortage has resulted in a scaled down version of London's Gay Pride event taking place today.
Organisers were forced to make cutbacks when they were unable to secure funding, blaming the "poor economic climate".
The procession set off from Portman Square in Westminster at 11am without floats and vehicles.
People have also been warned not to go to Soho expecting open air entertainment, as no street events have been planned, a Pride spokesman said.
The celebrations will finish in Trafalgar Square at 6pm, earlier than in previous years, organisers said.
The spokesman said that all the agencies involved had agreed that it was prudent to deliver an event that is affordable, without compromising the safety, security and the integrity of the event.
A statement on the Pride London website said: "London and UK's LGBT community has never been one to flounder in the face of adversity.
"Despite the changes in the past week, we urge all members of the community to come out in force and support WorldPride and do the very best they can.
"Forty years on from the very first Pride London march, we need to show that this fighting spirit from 1972 lives on. Not just for us, but for our brothers and sisters across the world who are still fighting for their rights to be human, their rights to love and the decriminalisation of homosexuality in the place that they live."
On Wednesday, charity chairman Dr Patrick Williams resigned from his position following criticism of the board's handling of World Pride 2012.
Pride London announced long-standing board member Tony Hughes as its interim chairman and said the rest of the board would remain unchanged and committed to delivering an event London can be proud of.
Gay human rights campaigner Peter Tatchell, who helped organise the first Gay Pride in Britain in 1972, said despite the problems and setbacks the numbers at today's events were huge and the atmosphere amazing.
He said: "It's much more political than in previous years. The global human rights message is really strong for lesbian, gay, bisexual, and transgender freedom.
"Nearly 80 countries still criminalise homosexuality with the penalties ranging from life imprisonment and even execution.
"We are also celebrating 40 years of Gay Pride in Britain. In those last four decades we have won the appeal of nearly every anti-gay law. All that remains is to win same sex marriage."
Prime Minister David Cameron said: "The UK has been judged to be the best country in Europe in which to live if you're gay so it is great that World Pride is being celebrated here in London - especially during this Diamond Jubilee and Olympic year.
"I'm very pleased that the Mayor of London has enabled the march and events in Trafalgar Square to go ahead and I want to thank all the volunteers who will be stewarding the event and contributing to it.
"It is 40 years since people first marched in London calling for equal rights. Since then we've come a very long way and progress is still being made. We have just finished consulting on how to introduce same sex marriage and we are working with countries across the globe to bring about greater equality.
"I hope you all have a happy Pride and remember all those who have, and those who are still fighting for, greater rights and protection for the LGBT community."