Throngs of Mexico City gay and lesbian couples registered for marriage licences Thursday, the day Latin America's first gay-marriage law took effect.
The first gay weddings will take place within a week to 10 days, after the paperwork is processed.
Mexico City's legislature approved the first law in December explicitly giving gay marriages the same status as heterosexual ones.
The change will allow same-sex couples to adopt children, apply for bank loans together, inherit wealth and be included in the insurance policies of their spouse, rights they were denied under civil unions allowed in the city.
"This is great, it is a feeling of relief, of celebration, everything," said Daniel Ramos, 20, a medical student planning to marry his boyfriend, Temistocles Villanueva, on March 12.
"For Latin America, this is not only a precedent, but an example to follow," he added.
For now the law applies only to residents of Mexico City, though a marriage performed in one Mexican state must be recognized in the rest of the country.
While activists are trying to get the law extended to the rest of Mexico's 31 states, conservatives say they will seek to pass a constitutional reform so that other states won't have to recognize marriages that do not conform to their laws.
For the time being, it appears easy to circumvent the residency requirement, because the city accepts a phone or utility bill as proof of address, often even if the bill is in someone else's name.
Activist Jaime Cobian showed up Thursday with a sheaf of required documents, birth certificates, official identification and residency documents, in a bid to get marriage licences for 16 gay and lesbian couples from states where no such law exists.
"What we still have to do is take this battle to all the states in the country," Cobian said.
An Argentine couple participated in Latin America's first gay wedding in December. But interpretations vary on whether Argentine law allows same-sex unions, and the question is now before that country's Supreme Court.
Argentina's constitution is silent on whether marriage must be between a man and a woman, effectively leaving the matter to provincial officials, who approved the wedding.
A law specifically legalizing gay marriage has been stalled in Argentina's congress since October.
The Mexico City law is being challenged by the federal government in Mexico's Supreme Court on constitutional grounds, but remains in effect while the appeal is heard.
The Roman Catholic Church has criticized the new law, which allows same-sex couples to adopt children, something several couples said they were thinking of doing.
David Razu, the Mexico City legislator who proposed the law, said he is confident the Supreme Court will uphold the law.
"There is always a wave of reaction to these kinds of measures, but we are prepared to face it," said Razu, a member of the leftist Democratic Revolution Party.
Gay couples entered the sprawling city government building and submitted their documents to the clerks amid hundreds of heterosexual couples doing the same thing.
Patria Jimenez, 54, waved a gay pride flag outside the building, shouting, "Freedom! At last, we have freedom!"
But, she said, she must overcome other, more personal obstacles before she can register for her own marriage licence.
"I still have not convinced my companion. I don't know if she will say yes," she said.