MEXICO CITY » Enrique Pena Nieto took the oath of office as Mexico’s new president today, bringing the old ruling party back to power after a 12-year hiatus amid protests inside and outside the congressional chamber where he swore to protect the constitution.

“I promise to uphold the constitution and its laws and to carry out loyally and patriotically the job of president of the republic,” he said with arm outstretched. “If I don’t, the people will demand it.”

The chamber erupted in cheers, with congress members chanting “Bravo president!” and “Mexico!” despite the earlier violent conflict, which had dispersed by the time the new president arrived.

Protesters opposed to Pena Nieto and the return of his Institutional Revolutionary Party, or PRI, had clashed with tear gas-wielding police. At least two 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.” The PRI ruled for 71 years with what many considered to be an iron fist, using a mix of populist handouts, graft and rigged elections.

“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.

After the oath-taking, Pena Nieto headed to deliver an inaugural speech at the historic National Palace in the city’s downtown. The new president also 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.

The protesters trailed, 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.

Associated Press writers Adriana Gomez Licon, Michael Weissenstein and Carlos Rodriguez contributed to this report.