Felipe Calderon has taken over as Mexico's president after an unusual midnight ceremony at the presidential residence in Mexico City.
Felipe Calderon says he will defy protests to be sworn in Congress
He replaces outgoing President Vicente Fox, a fellow conservative.
Mr Calderon has said he is determined to be formally sworn in later on Friday despite a threat of disruption from opposition politicians.
Members of the left-wing party, the PRD, say Mr Calderon won July's presidential election by fraud.
He defeated the left-wing candidate, Andres Manuel Lopez Obrador, by less than a percentage point.
Mr Obrador last week launched a "parallel government" in Mexico City and held an unofficial swearing-in ceremony.
In a live television broadcast from the presidential residence, Mr Calderon appealed for an end to the divisions that have dogged Mexico since the disputed elections.
Mr Lopez Obrador declares himself the rightful president of Mexico
"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 we today should put an end to our disagreements and from there, start a new stage whose only aim would be to place the interests of our nation above our differences."
Mr Fox, who was present at the midnight ceremony, handed over the presidential sash to a military officer.
Mr Calderon will receive it in the inauguration ceremony in Congress later on Friday morning.
The BBC's Duncan Kennedy in Mexico City says the effective handing over of power with only a few invited guests, behind closed doors and late into the night, will have shocked many Mexicans.
Opposition politicians have been staging a sit-in in Congress
The official inauguration is still expected to go ahead in front of dozens of current and former world leaders in Congress, our correspondent says.
But there is some uncertainty about the event because the chamber is being partially occupied by opposition lawmakers who have been staging a sit-in for the past few days.
The room was earlier this week the scene of scuffles between opposition deputies and Mr Calderon's supporters.
There are concerns more protests may disrupt the swearing-in ceremony.
Mr Lopez Obrador's followers are also due to hold a rally to mark their continued opposition to Mr Calderon.
All this will worry but not deter Mr Calderon, our correspondent says.
The new president has outlined a series of policies intended to unite a divided country and has said he will make jobs his top priority.
This will mean strengthening his ties with the US, our correspondent says. Relations have been strained in recent months over plans by Washington to build a fence along the two countries' common border.
Mr Calderon will also have his hands full cracking down on crime, introducing more competition to the economy and quelling a virtual uprising in the southern city of Oaxaca.
It promises to be a deeply controversial and highly-charged start to his presidency, our correspondent concludes.