Enrique Pena Nieto has been sworn in as Mexico's new president, promising to boost economic growth and tackle drug-related violence.
"My government's first aim will be to bring peace to Mexico," he said.
Some 60,000 people were killed in drug-related violence during the rule of his predecessor, Felipe Calderon.
Security was tight in Mexico City for the inauguration earlier as petrol bomb-wielding protesters clashed with riot police who fired tear gas.
One protester was gravely injured after being hit with a tear gas canister.
The demonstrators are angry at what they say was vote-buying by the president's campaign, says the BBC's Will Grant in Mexico City.
They were also protesting more generally against the return to power of Mr Pena Nieto's Institutional Revolutionary Party (PRI) after a 12-year absence.
But in his inaugural speech at a ceremonial palace in the old city centre, Mr Pena Nieto said the state had "lost ground" since his party left power.
"Lawlessness and violence have robbed various parts of the country of peace and freedom," said the 46-year old.
Earlier, amid a cacophonic trumpet fanfare, outgoing President Felipe Calderon handed a sash with the colours of the Mexican flag to Mr Pena Nieto at the city's Congress building.
All week in Mexico City, a ring of steel had been in place around the building ahead of the ceremony.
It had created massive traffic jams in the already congested city as authorities tried to prevent protesters from gaining access to parliament.
Earlier, the new president, who is 46, named a 20-member cabinet.
His foreign minister will be Jose Antonio Meade, who was treasury minister in Mr Calderon's outgoing government.
Mr Pena Nieto spent his final days as president-elect in talks with leaders in the US and Canada, Mexico's partners in the regional trade organisation, Nafta.
The PRI governed without interruption for 71 years until 2000, and its opponents often accused it of being authoritarian and corrupt, and of having links to the drug cartels.
But during his election campaign Mr Pena Nieto was adamant the party had evolved.
"I can say categorically that in my government, there won't be any form of pact or agreement with organised crime," he told the BBC. "It's not the path nor the route to greater security for the Mexican people."
Earlier this month, he denied the PRI's return to power would be a return to the past. "It is not, because this is a different country," he said.
The new president has indicated he will focus on tackling extortion, kidnapping and murder, and he has drafted in support from Colombia's former top policeman.
The outgoing president is due to take up a position at Harvard University and maintains he has made Mexico safer.
But for many Mexicans, Mr Calderon's legacy as president is of an estimated 10,000 drug-related killings a year for the past six years.