Felipe Calderon took power as Mexico's president on Friday and pleaded for an end to months of unrest led by leftists who claim he stole July's election and have vowed to block him from taking the oath of office.
Calderon replaced outgoing President Vicente Fox, an ally and fellow conservative, in a solemn midnight ceremony at the presidential residence in Mexico City.
But he was set to face a hostile reception from left-wing lawmakers later on Friday when he was to take the oath of office and put on the presidential sash at his formal inauguration in Congress.
"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 brief speech to the nation after hugging and shaking hands with Fox.
The bitterly contested July 2 presidential election has split Mexico, with left-wing parties claiming it was rigged.
Andres Manuel Lopez Obrador, the fiery anti-poverty campaigner who narrowly lost the vote, has drawn hundreds of thousands of supporters onto the streets.
Although Mexico's top electoral court threw out the fraud claims, Lopez Obrador claims to be "legitimate president" and will lead a new protest on Friday in the capital's vast central square.
Conservative and leftist lawmakers punched and shoved each other in an ugly brawl in Congress earlier this week and both sides have camped out there since, with Lopez Obrador's allies insisting they will not let Calderon in for his inauguration.
Despite the threat of fresh violence, Calderon refused to back down and insisted he would go to Congress.
He called on his opponents to respect the law and let him be sworn in.
Former US President George Bush and Spain's Crown Prince Felipe are among the few prominent foreign dignitaries expected at the inauguration, set to start at 9:45 am.
Calderon's election victory bucked the trend of left-wing gains across Latin America in recent years and he will be a key ally of the United States in the region.
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.
Calderon faces other serious challenges in trying to assert control over an increasingly violent country.
A vicious war between rival drug-smuggling gangs has killed nearly 3,000 people in the last two years, and the popular tourist city of Oaxaca has been wrecked by six months of violent street protests against a state governor.