Scuffles and insults reined as the two leading parties took over the main dais at the Chamber of Deputies and vowed not to leave before Friday morning, when Felipe Calderón is scheduled to be sworn in as the next president.

Several deputies were injured in the sporadic shoving and blows as legislators from the Democratic Revolution Party (PRD) and the National Action Party (PAN) jockeyed for position around the several speaker´s podiums at the front of the Chamber.

Presiding office Jorge Zermeño, after repeated efforts to restore order, finally declared an end to the session just before 3 p.m., almost two hours after the unrest began. The legislators will not meet again until they are convened at 9:30 a.m. on Friday for the inauguration ceremonies.

The PRD has vowed to impede Calderón´s inauguration, insisting that his victory in the July 2 presidential election was fraudulent. PAN deputies, suspecting some kind of preliminary disruptive action by the PRD Tuesday, left their seats en masse in an apparent pre-emptive storming of the speakers´ area.

PRD legislators quickly joined them, creating a bizarre scene of scores of lawmakers from competing parties jostling for position as they lined up in rows like a church choir. Each faction held up signs and took turns chanting, "Felipe will fall" coming from the PRD and "Felipe, president!" from the PAN.

"We very much regret that people had to see this, but we´re defending a man whose name is Felipe Calderón," said Adriana Dávila Fernández, a PAN deputy from the state of Tlaxcala. "We´ve been quiet and even accused of being too passive, but enough is enough."

Zermeño called a special meeting of the coordinators of each party´s legislative contingent in a last-ditch effort to reach an agreement before Friday´s event. The ceremony will be attended by visiting dignitaries and heads of state, including former U.S. President George H.W. Bush.

While the PAN and the PRD battle it out, the once-powerful Institutional Revolutionary Party (PRI) has been positioning itself as the voice of reason.

"I think the time has come when all of us need to act with prudence," said Manlio Fabio Beltrones, a powerful senator with the PRI, which finished third in the presidential race and dropped to the third force in both houses of Congress. "The nation is in a very tense political situation and we must not make it worse."

Beltrones and other PRI leaders have advocated changing both the site and format of Calderón´s swearing-in ceremony, precisely to avoid the kind of disturbance that took place Tuesday. Calderón has rejected any change of venue, insisting that he will take his oath before Congress as his predecessors have done.

Beltrones, however, held out hope that an alternative plan can still be worked out.

"I think there´s still time to change course," he said, responding to questions about Tuesday´s events at the Chamber of Deputies. "There are many other ways we can comply with our constitutional obligations."

Legal experts agree that Calderón automatically becomes president as of one second after midnight on Dec. 1. But the Constitution also requires him to take an oath of office, as all other elected officials must do.

There is some dispute about whether that oath must be taken before a joint session of Congress, as is usually done.

Santiago Creel, leader of the PAN in the Senate, said Tuesday night that Calderón must and will be sworn in at Congress, and asked the PRI to drop its efforts to force a change of venue.

But Emilio Gamboa Patrón, PRI coordinator in the lower house, said more than 80 percent of PRI deputies are in favor of keeping Calderón away from the Chamber.