Felipe Calderon took the oath of office as Mexico's president Friday amid jeers and whistles, in a chaotic ceremony before congress preceded by a brawl between lawmakers still divided over the nation's tight presidential election.

Physically protected by ruling party lawmakers and flanked by outgoing President Vicente Fox, Calderon quickly swore to uphold the constitution. The national anthem was then played, momentarily stilling the catcalls and shouting. Calderon swiftly left the chamber as congress adjourned.

"He did it! He did it!" chanted ruling party lawmakers.

Earlier, the conservative Calderon took control of the presidential residence in an unusual midnight ceremony with Fox, swearing in part of his Cabinet. In that private ceremony, broadcast live from Los Pinos, Fox handed the presidential sash to a military cadet as his term ended at midnight.

That left experts on Mexico's constitution, which requires presidents to be sworn in "before congress," puzzled over whether Mexico had a president or not earlier Friday morning.

Leftist lawmakers threw punches and chairs at their conservative colleagues and some tried to block the doors of the congressional chamber an hour before the swearing in ceremony was scheduled to begin.

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Ruling party lawmakers, chanting "Mexico wants peace," seized the speaker's platform, while leftist opponents blocked most of the chamber's doors.

The brawl was shown on live television across Mexico.

In the midnight broadcast, Calderon called on Mexicans to leave behind the divisions that have dogged him and the country since the disputed July 2 elections.

"I am not unaware of the complexity of the political times we are living through, nor of our differences," he said. "But I am convinced that today we should put an end to our disagreements and from there, start a new stage whose only aim would be to place the interests of the nation above our differences."

The leftist Democratic Revolution Party — whose candidate, Andres Manuel Lopez Obrador, narrowly lost the presidency to Calderon — massed thousands of supporters nearby in the capital's main Zocalo plaza and pledged to march peacefully to the National Auditorium.

Lopez Obrador, refusing to recognize Calderon's victory, has set up a parallel government of sorts and declared himself "legitimate president" of Mexico.