The National Council of Justice, which oversees the Brazilian judicial 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.
"This is the equivalent of authorizing homosexual marriage in Brazil," said Raquel Pereira de Castro Araujo, head of the human rights committee of the Brazilian bar association.
The Brazilian Congress, where a strong religious faction opposes same sex marriage, has not yet approved a law legalizing gay marriages. And the council's decisions are subject to appeal before the Supreme Court.
But Supreme Court Chief Justice Joaquim Barbosa said there was no reason for the government's marriage licensing offices to wait for the Brazilian Congress to pass a law authorizing same-sex marriage before extending the right to gays.
He noted that the Supreme Court in 2011 recognized stable homosexual unions, ruling that the constitution guaranteed them the same rights as heterosexual couples.
"Are we going to require the approval of a new law by the Congress to put into effect the decision that was already taken by the Supreme Court? It makes no sense," he said in comments quoted by the G1 news website.
The Supreme Court decision "is binding" and should be followed by the lower courts, he said.
Some offices have granted marriage licenses to gay couples and others have not. While some state courts have recognized same-sex marriages, the council's decision was the first to set a national standard.
"Since the Congress is so slow, and doesn't decide, the judicial branch took the lead," said Luiz Kignet, a specialist in family law at PLKC Advogados in Sao Paulo.
"The law is necessary, the judicial branch is not suppressing the obligation to have a law," he said.
But it is saying that same-sex marriage is constitutional, and the council's decision should accelerate the approval of a law formally authorizing homosexual marriage.
"When there is a law, everything is easier. The law regulates concrete cases for everyone," he said.
In theory, the council's decision could be challenged by the Supreme Court, but it is not likely to, said Kignet, saying it had reached a point of no return.