Pena Nieto, marking the return of the institutional Revolutionary Party, or PRI, promised everything from a new integrated program to prevent crime to ending the patronage and buying of teacher positions that rule the public education system and opening up broadband Internet service now dominated by just a few telecommunication monopolies.
"It's time to move Mexico and to achieve a national transformation," Pena Nieto said. "This is the moment for Mexico."
The return of the PRI after a 12-year hiatus started with violent confrontations in the streets and protest speeches from opposition parties inside the congress, where Pena Nieto took the oath of office. Protesters continued vandalizing downtown businesses, smashing plate glass windows and setting office furniture ablaze outside.
Protesters clashed with tear gas-wielding police, calling the inauguration of Pena Nieto an "imposition" of a party that ruled with a near-iron fist for 71 years using a mix of populist handouts, graft and rigged elections. At least two people were injured, one gravely, police said, and a police officer who was bleeding from the face was taken for medical treatment
Leftist congress members inside the chamber gave protest speeches and hung banners, including a giant one reading, "Imposition consummated. Mexico mourns."
One word sums up Dec. 1: The restoration. The return to the past," said Congressman Ricardo Monreal of the Citizens Movement party.
But PRI leaders denied that.
"This is a time of expectation, a time of hope," said PRI Senator Omar Fayat. "What we did well in the previous government we will preserve and strengthen. What we didn't, we will rebuild and reorient."
Pena Nieto had taken over at midnight in a symbolic ceremony after campaigning as the new face of the PRI, repentant and reconstructed after being voted out of the presidency in 2000.
Before his public swearing in later in the morning, hundreds banged on the tall steel security barriers around Congress, threw rocks, bottle rockets and firecrackers at police and yelled "Mexico without PRI!" Police responded by spraying tear gas from a truck and used fire extinguishers on flames from Molotov cocktails. One group of protesters rammed and dented the barrier with a large garbage-style truck before being driven off by police water cannons.
"We're against the oppression, the imposition of a person," said Alejandro, 25, a student and protester who didn't want to give his last name for fear of reprisals. "He gave groceries, money and a lot more so people would vote for him," the student said, referring to allegations that the PRI gave voters gifts to encourage them to cast their ballots for Pena Nieto.
Another banner inside the chamber read: "You're giving up a seat bathed in blood," referring to outgoing President Felipe Calderon's attack on organized crime and the deaths of 60,000 people during that six-year offensive by some counts.
Despite the protests, the swearing-in atmosphere at Congress was far less chaotic than six years ago, when a Calderon security unit literally had to muscle him past blockades and protesters to get him into Congress so he could take the oath of office after a razor-thin, disputed victory over a leftist candidate.
Calderon had worked hard for a smooth transition after that experience.
But vandalism and rock throwing continued throughout the day near the city center where Pena Nieto gave his inaugural speech.
Later in the afternoon, the new president planned a luncheon for invited guests, including U.S. Vice President Joe Biden, Prince Felipe of Spain and the presidents from Colombia, Peru, Honduras and Nicaragua, among other Latin American countries.
Lines of riot police closed down streets around the Palacio de Bellas Artes, the Fine Arts Palace. Police arrested a few protesters who were throwing rocks or pieces of wood at them. An AP reporter saw one of the windows of the Sears departmental store was smashed and the building marred with large splashes of white paint.
Protesters trailed the new president from the Congress to the National Palace, shouting, "Murderers, murderers!" and trying to break the barriers set up in the Zocalo, Mexico City's giant central plaza where the palace is located.
"The president is like Salinas, 'I don't see you, I don't hear you,' " said Aurelio Medina, 64, referring to PRI President Carlos Salinas de Gortari.
Down the street at the Alameda Park, other protesters hung thousands of handkerchiefs with the names of drug-war victims.
"It's a very important day for us to take stock of the damage. They have first and last names," said Regina Mendez, a member of the group Embroidering for Peace.
Pena Nieto has promised to govern democratically with transparency. But his first moves even before the inauguration showed a solid link to the past. In announcing his Cabinet on Friday, he turned to the old guard as well as new technocrats to run his administration.
He also pledged to make economic growth and job creation the centerpiece of his administration, with campaign manager and long-time confidant Luis Videgaray the point person. Videgaray, a 44-year-old economist with a doctorate from the Massachusetts Institute of Technology, will lead the treasury department.
Miguel Angel Osorio Chong, a 48-year-old former state governor who is known as a political operator and deal maker, has been named secretary of the interior, a post that will play a key role in security matters.
Pena Nieto has also promised to push for reforms that could bring major new private investment into Petroleos Mexicanos, or Pemex, the crucial but struggling state-owned oil industry. Such changes that have been blocked for decades by nationalist suspicion of foreign meddling in the oil business.