Felipe Calderon took over as Mexico's president on Friday and pleaded for an end to months of unrest over his narrow election win, but a huge brawl erupted in Congress where leftist opponents vowed to block him from taking the formal oath of office.
Dozens of rival lawmakers threw punches and chairs at each other on the floor of Congress on Friday morning, less than two hours before Calderon was to be sworn in.
He replaced outgoing President Vicente Fox, an ally and fellow conservative, in a solemn midnight ceremony at the presidential residence in Mexico City.
Mexico's constitution requires that he takes an oath of office at a formal inauguration in Congress, however, and left-wing opponents who say he stole July's election with fraud planned a hostile reception for him.
"I do not ignore the complexity of the political situation or our differences, but I am convinced that today we must put an end to our disagreements," Calderon, 44, said in a midnight speech after hugging Fox and swearing in key cabinet members.
The bitterly contested July 2 presidential election has split Mexico, with left-wing parties claiming it was rigged. Conservative and leftist lawmakers attacked each other in an ugly brawl in Congress earlier this week, and fighting broke out again on Friday morning.
Leftists built barricades of chairs to block the main doors into the lower house Chamber of Deputies. "Calderon is not coming in here, we are going to stop him," said deputy Adriana Diaz.
"He is not coming in the front entrance. Thieves come in through the back door." Presidential guards earlier used sniffer dogs and metal detectors to sweep the floor for explosives.
Andres Manuel Lopez Obrador, a fiery anti-poverty campaigner who narrowly lost the July election, has drawn hundreds of thousands of supporters onto the streets in peaceful protests over the past five months.
Although a top electoral court threw out his fraud claims, Lopez Obrador says he is "legitimate president" and planned to lead a new protest on Friday in the capital's vast central square.
Thousands massed in the square before dawn, chanting and waving banners as more arrived in buses.
Calderon insisted he would go to Congress and called on his opponents to respect the law and let him be sworn in.
Former U.S. President George Bush, father of the current U.S. president, and Spain's Crown Prince Felipe are among the few prominent foreign dignitaries here for the inauguration.
Calderon will be a key ally of the United States in Latin America, which has turned away from Washington in recent years with a string of left-wing gains in presidential elections.
A career politician who has an iron will but little charisma, he will also try to push pro-business reforms through Congress and keep a tight rein on government spending even as he promises to cut the vast gap between rich and poor.