Incoming President Enrique Pena Nieto of Mexico was inaugurated on Saturday amid tight security in the capital Mexico City.
He wants to boost economic growth and cut drug-related violence.
About 60,000 people were killed in drug-related violence during the rule of his predecessor, Felipe Calderon.
As Mr Pena Nieto's motorcade approached Congress, petrol bomb-wielding protesters clashed with riot police who fired tear gas outside the building.
One protester was gravely injured after being hit with a tear gas canister.
The demonstrators were angry at what they say was vote-buying by the president's campaign.
Mr Pena Nieto has indicated he will focus on tackling extortion, kidnapping and murder.
"The state has lost ground in important areas. Lawlessness and violence have robbed various parts of the country of peace and freedom," he said in his inaugural speech.
"My government's first aim will be to bring peace to Mexico."
Mr Pena Nieto now wants to pass legislation to strengthen Mexico's tax base and allow more private investment in Pemex, a state oil company.
"Mexico has not achieved the advances the people demand and deserve," he said.
"We are a country growing at two speeds. There's a Mexico of progress and development. But there's another one too that's been left behind in poverty."
If he is successful, the reforms could help spur stronger growth and create jobs.
Mr Pena Nieto, 46, a former governor of the State of Mexico won the election on 1 July with about 38% of the vote.
The BBC reports he spent his final days as president-elect in talks with leaders in the United States and Canada, Mexico's partners in the regional trade organisation, NAFTA.
Mr Calderon is to take up a position at Harvard University.