Antonio Medina (L) said exclusion had come to an end

The law, which came into effect in the capital on Friday, gives gay couples similar social and inheritance rights to heterosexual couples.

Civil unions were approved by the city council in November despite opposition from the Roman Catholic Church.

Some MPs of President Felipe Calderon's conservative party are filing a court challenge against the gay unions.

They argue that the partnerships violate constitutional protection of the family.

'Plurality'

Among the first Mexico City same-sex couples to tie the knot were journalist Antonio Medina, 38, and economist Jorge Cerpa, 31.

Mr Medina said: "With this law, a history of exclusion comes to an end. Today, the love that before did not dare speak its name has now entered the public spotlight."

Although the capital was the first to pass the same-sex law in Mexico, similar legislation came into effect earlier in the northern border state of Coahuila.

A lesbian couple registered their union on 31 January.

Julio Cesar Moreno, a local councillor who oversaw another union in Mexico City on Friday, said: "The era of plurality and diversity is permeating Mexico City."

The new laws do not afford same-sex couples all legal marital rights, particularly on adoption.