Felipe Calderon has taken over as President of Mexico in an unusual midnight ceremony held in an attempt to thwart protests by left-wing demonstrators.
Mr Calderon also swore in some of his staff and said he would not be deterred from taking the formal oath of office before Congress today.
Mr Calderon's opponents, who are allied with Andres Manuel Lopez Obrador, Mr Calderon's left-wing opponent in the recent presidential election, have occupied the podium where the swearing-in ceremony is scheduled to take place.
In a live broadcast from the presidential residence of Los Pinos, Mr Calderon called on Mexicans to leave behind the divisions that have dogged him and the country since the July 2 vote. He takes over from Vicente Fox, who handed over his presidential sash as his term ended at midnight.
The bitterly contested July 2 presidential election has split Mexico, with left-wing parties claiming the result was rigged.
Mr Obrador, a fiery anti-poverty campaigner who narrowly lost the vote, has drawn hundreds of thousands of supporters onto the streets.
Although a senior electoral court dismissed his accusations of fraud, Mr Obrador claims he is the "legitimate president" and is planning to lead a protest today in the capital's vast central square.
Conservative and left-wing representatives 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 Mr Calderon in for his inauguration.
Despite the threat of fresh violence, Mr Calderon has 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.