Under rainy skies and cold that did not dampen many spirits, marchers waved banners like "Never going back in the closet" in the financial and industrial hub of this majority Roman Catholic country of about 196 million.
"We have got to make a change in the Constitution so that we are all citizens with the same rights," said Culture Minister Marta Suplicy, a former Sao Paulo mayor and gay rights supporter, drawing cheers from the crowd.
In May, Brazil became the third and largest Latin American country to give a de facto green light to same-sex marriage.
In a bold stride for the majority Roman Catholic nation, the National Council of Justice (NCJ), a panel which oversees Brazil's legal system and is headed by the chief justice of the Supreme Court, said government offices that issue marriage licenses had no standing to reject gay couples.
Since 2011, some offices have granted marriage licenses to gay couples but others have not.
While some state courts have recognised same-sex marriages, the council's ruling was the first to set out a national standard.
In Congress, a strong religious faction opposes same-sex marriage, and has not yet approved a law on same-sex marriage regulations. And the NCJ's decisions are subject to appeal before the Supreme Court.