Lawmakers wrestled, slapped each other and tumbled across the floor of Mexico's Congress after opposition legislators threatened to block the inauguration of the incoming president, whom they accuse of stealing the election.

By late Tuesday, the brawl had turned into a tense standoff between congressmen of President-elect Felipe Calderon's conservative party — who want him to take the oath of office in Congress — and opposition leftists who have vowed to block the swearing-in ceremony.

Click here to view a photo essay of the brawl.

The battle showed how hard it could be for Calderon to unite a nation divided since he narrowly defeated opposition candidate Andres Manuel Lopez Obrador in the disputed July 2 election.

Congress has seen plenty of degrading behavior, but Tuesday's brawl came as Mexico faces central questions on the effectiveness of its government, with escalating turf wars between drug gangs and bloody street battles in the southern city of Oaxaca, which was seized for five months by leftist protesters.

Calderon has pledged to reach out to the millions of people who didn't vote for him by building a coalition government that will include several of his rival's proposals to help the poor. But so far, he has stacked his Cabinet with militants from his own party.

The congressional chaos began after conservative legislators of Calderon's National Action Party, or PAN, took over the speaker's podium early in the day amid rumors that leftist lawmakers planned to seize Congress, as they did before President Vicente Fox's Sept. 1 state-of-the-nation speech.

Leftists from Lopez Obrador's Democratic Revolution Party, or PRD, quickly followed, and scuffles broke out.

Tired and bedraggled, the lawmakers ate fast food and occasionally broke out into fresh bursts of shoving and shouting late into the night. Nearly everybody in Mexico's political scene found the spectacle depressing.

"I'm sorry this had to happen, but we were forced to do it," said PAN Rep. Juan Jose Rodriguez Pratts, whose colleagues occupied the upper steps of the broad, raised speaker's podium, while opposition legislators formed an angry knot on the bottom steps.

Rodriguez Pratts said PAN lawmakers felt they had to take action to ensure that the inauguration would not be blocked by leftists.

"There were clear indications, latent threats to do that, and so what we did was head that off to guarantee Friday's ceremony," said Rodriguez Pratts.

The former ruling Institutional Revolutionary Party called the spectacle "shameful," and one of its leading members, Sen. Manlio Fabio Beltrones, suggested it would be "natural and logical" to simply hold the inauguration ceremony elsewhere if the congressmen refuse to leave before Friday.

"Our colleagues are going to stay there on the podium for the next 72 hours," vowed PRD national leader Leonel Cota.

Disputes over the July 2 elections are unlikely to go away. Lopez Obrador says he was cheated of the presidency by vote fraud, has declared himself the "legitimate president of Mexico," endorsed and led street protests and has refused to recognize or accept Calderon, whom he calls "the lackey" and "the spurious president."

On Tuesday, Calderon named Francisco Ramirez Acuna as his interior secretary, the government's No. 2 post in charge of domestic security and political affairs. The former Jalisco governor has been criticized for turning a blind eye as police detained dozens of protesters during an international summit in his state in 2004. In a report last week, the U.N. Committee Against Torture expressed concern about the arrests.

The U.S. Embassy said Tuesday that former President Bush will attend the inauguration of Calderon, who has promised to crackdown on drug trafficking and maintain close ties with the U.S.