Buenos Aires City legislators have ordered Mayor Anibal Ibarra to face impeachment over a fire that killed nearly 200 people last year.
Mr Ibarra is an ally of Argentina's President Nestor Kirchner
The legislature also decided to suspend Mr Ibarra, who denies any responsibility in the tragedy.
The blaze at the Republica Cromanon nightclub started when someone fired a flare during a concert. It was one of Argentina's worst disasters.
Some victims' groups blame Mr Ibarra's government for poor safety standards.
The impeachment process, approved by a narrow vote, will be conducted by a legislative commission and will carry no criminal punishment.
Mr Ibarra's deputy, Jorge Telerman, will take over the Buenos Aires government during Mr Ibarra's suspension, which is due to last four months.
The suspended mayor - an ally of centre-left President Nestor Kirchner - said he would not step down and described the effort to unseat him as "an institutional coup" by conservative opponents.
Dozens of relatives of the victims, waiting in the streets outside the city legislature, cheered and hugged each other as they learned of the decision.
Police officers had sealed off the building to prevent rioting.
Last week, violence erupted when families attending a session were upset by the decision to break off the debate without a vote on the mayor's impeachment.
There have been several protests against the local government since the fire occurred in December last year.
Relatives blame poor safety standards
Republica Cromanon was reported to have been filled beyond its capacity and some of its emergency exits were said to have been locked. About 700 people were injured.
Most of the victims were young people.
Two Buenos Aires officials stepped down after accepting responsibility for inadequate safety measures.
But some groups want Mr Ibarra to resign, accusing him of maladministration.
He has refused to quit, saying that the fire brigade was responsible for such safety issues in Buenos Aires and that it had approved the nightclub.
He has also blamed the club's owner, Omar Chaban.
An Argentine judge has charged Mr Chaban with manslaughter. He faces up to 25 years in prison.