Homemade bombs exploded early Monday at the Federal Electoral Tribunal, a bank branch and the headquarters of the former ruling party in the country's capital, causing no injuries but rattling nerves in a country wracked by protests since a contested presidential vote.

A coalition of resistance groups claimed responsibility, but officials said it still wasn't clear who carried out the blasts.

The explosions shortly after midnight damaged an auditorium at the headquarters of the Institutional Revolutionary Party, or PRI, a branch of Canadian-owned Scotiabank, the Federal Electoral Tribunal, and businesses and residences near the court.

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Police deactivated two other bombs, one at a second Scotiabank near the court and another outside a Sanborns restaurant, a chain owned by billionaire Carlos Slim, near the PRI headquarters, said Mexico City Public Safety Secretary Joel Ortega.

Authorities said the deactivated bomb at the Scotiabank was inside a box labeled "Bomb-Danger."

Ortega said emergency officials received an anonymous call warning that bombs were about to be detonated.

Five leftist resistance groups said they carried out the blasts in support of a monthslong protest movement in the conflict-torn southern state of Oaxaca.

"We take full responsibility for these actions," the groups said in a statement e-mailed to the news media, which included the name of each group.

But federal attorney general's office spokesman Jose Luis Manjarrez said it still wasn't clear who was behind the blasts.

"There is nothing at this point to be able to confirm the veracity of the statement," Manjarrez told The Associated Press. Police "are continuing their investigations but at this moment there is no clear report of who may have been responsible."

Authorities and political parties condemned the bombings, with some saying they were probably carried out by groups trying to destabilize the government before President-elect Felipe Calderon's swearing-in on Dec. 1. Calderon is a member of Fox's National Action Party.

"We categorically reject these criminal acts aimed at frightening the population, and we're going to work vigorously to clear this up and guarantee security," President Vicente Fox said.

Mexico City Mayor Alejandro Encinas asked residents not to panic but acknowledged the blasts were "creating a climate of uncertainty."

Mexico City's police department intensified security in the city's public transportation system, as well as at the presidential residence, Los Pinos, several federal government offices and at the U.S. and British Embassies, said a spokesman for the department, who was not authorized to give his name.

The explosions came a day after more than 20,000 leftists from across Mexico marched in Oaxaca to demand the withdrawal of federal police who were sent in on Oct. 29 to end violence linked to a five-month protest against the state's governor.

The protesters are seeking the resignation of Oaxaca state Gov. Ulises Ruiz of the Institutional Revolutionary Party, or PRI, whom they accuse of rigging the elections that brought him to power in 2004 and oppressing dissent.

Flavio Sosa, a protest leader, said his movement had no ties to the explosions and did not know who could be behind them.

The PRI, which ruled Mexico for 71 years before Fox's 2000 triumph, backed the electoral tribunal when it confirmed Calderon's victory by less than 1 percentage point over leftist Democratic Revolution Party candidate Andres Manuel Lopez Obrador.

Lopez Obrador, a former Mexico City mayor, cried fraud and launched a massive protest that clogged the capital for more than a month to demand a recount, which the court refused to order.

Lopez Obrador and the Democratic Revolution Party have expressed support for the Oaxaca protesters, but party president Leonel Cota denounced the blasts.

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