The Prime Minister is to welcome equality rights campaigners to Downing Street before the annual Pride march through central London.
Gordon Brown's wife, Sarah, is expected to join the colourful celebration of lesbian, gay, bisexual and transgender culture later.
In his message to Pride London, Mr Brown described the creation of civil partnerships as one of a set of "massive strides towards equality" for the gay community made under the Labour Government which were made "often in the face of fierce opposition".
"This Government is committed to standing at your shoulders in the fight for equality and we are guided by one very simple principle when it comes to LGBT rights: you can't legislate love," he said.
Pride founder and prominent gay rights campaigner Peter Tatchell, who is expected to march alongside Mrs Brown, on Friday described civil partnerships for same-sex couples as "a form of sexual apartheid" because they institutionalised different marriage laws for heterosexual and homosexual people.
He said he hoped to persuade Mrs Brown to talk to the PM about it.
The celebration follows a political row which on Friday saw Labour accused of "poisonous mudslinging" by Alan Duncan, the shadow leader of the House of Commons.
Mr Duncan said two senior ministers, both openly gay like him, were engaged in a co-ordinated attempt to stir up hatred and reopen old divisions.
He was responding to remarks by Culture Secretary Ben Bradshaw at a public debate that there remained a "deep strain of homophobia" running through the Conservative Party.
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