More than 250 groups took part in the parade, which started in Baker Street at 13:00 BST.
Police said there was extra security on the route following terror attacks in France, Tunisia and Kuwait on Friday.
The United States flag joined Ireland and Mozambique's at the front of the march, recognising recent gay rights progress in the three countries.
On Friday, the US Supreme Court ruled same-sex marriage was a legal right across the United States, meaning 14 states with bans would no longer be able to enforce them.
Gay marriage is legal across Great Britain but not in Northern Ireland.
Local road closures were in place for this year's Pride parade route, which finished in Whitehall.
The Met's Deputy Assistant Commissioner Helen Ball said they had laid on extra security "to help protect and reassure the public".
In a statement, the force said: "While the UK threat level from international terrorism remains severe, we would like to reassure the public that we constantly review security plans for public events, taking into account specific intelligence and the wider threat.
"Our priority is the safety and security for all those attending or involved. The public are encouraged to continue with their plans to attend or take part in events as normal."
Last year more than 750,000 people attended Pride In London.
Organisers of the event said earlier this month they had to "wrestle with a difficult issue" when deciding whether or not to allow a group from the political party UKIP to enter the parade.
However, they said the decision was made to reject the application "in order to protect participants and ensure the event passes off safely and in the right spirit".
Thousands of people have taken to the streets of London for the annual Pride parade.