"All forms of marriage deserve respect, dignity and the support of the state," said President Sebastian Pinera, who signed the proposal and sent it to Congress.

"This puts opposite-sex and same-sex couples on the same footing, because in both cases it is possible to develop love, affection and respect."

Pinera, who brought conservatives to power after 20 years of center-left rule in the country, grated on his own election campaign when he announced his intention to legalize civil unions for gay couples. He said two million people in Chile live together without marrying.

But the president has repeatedly stressed his opposition to gay marriage.

"I deeply believe that marriage is by nature between a man and a woman, but that conviction does not prevent me from recognizing that other forms of affective relationships exist," he said.

The law would permit gay couples who join into a civil union to have access to inheritance and other social benefits.

Its adoption would mark an important chapter for gay rights in Latin America, a mostly Catholic region where only a handful of countries recognize civil unions.

It will be a notable turn in Chile, where 80 percent of the country is Catholic and the government did not recognize divorce until 2004.

Leaders of the two majority parties, Carlos Larrain from the Renovation National and Juan Antonio Colona of the Independent Democratic Union (UDI) boycotted Pinera's ceremony unveiling the bill.

A deputy from the conservative UDI party, Gonzalo Arenas, lashed out at a government "capable of selling its mother for increasing its poll numbers," according to Radio Cooperativa.

"The president is not listening to members of parties that are his allies, but he is listening to students."

Pinera's announcement, which follows a campaign promise, comes as he faces record unpopularity of 26 percent since the return of democracy in 1990 and as student protests have roiled the country over the past three months.