Enrique Pena Nieto headed to Congress to take the oath of office as Mexico's new president on Saturday, bringing the old ruling party back to power after a 12-year hiatus amid protests inside and outside the chamber where he was to be sworn in.

As his motorcade approached Congress, protesters opposed to the new president 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.

Protest banners were hung inside the chamber, including a giant one reading, "Imposition consummated. Mexico mourns."

Pena Nieto had taken over at midnight in a symbolic ceremony after campaigning as the new face of the Institutional Revolutionary Party, or PRI, repentant and reconstructed after being voted out of the presidency in 2000. The PRI ruled for 71 years with a mix of populist handouts, graft and rigged elections.

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."

Promises transparency

Before the swearing in, leftist congressmen inside the chambers demanded to know what was going on in the crowds and urged police to use restraint. Rumours circulated that one had died, but police denied that there were any deaths. Congressman Rafael Huerta of the Labor Party urged the new government to protect the people.

Another banner 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 in that offensive by some counts.

Despite the protests, the swearing-in atmosphere at Congress was far less chaotic than six years ago, when 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 his experience.

After the oath-taking, the new president planned to deliver an inaugural speech at the historic National Palace in the city's downtown. Pena Nieto also planned a luncheon for invited guests, including U.S. Vice-President Joe Biden, Prince Felipe of Spain and Colombian President Juan Manuel Santos.

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.

Protesters accuse Enrique Pena Nieto's party, the PRI, of buying votes. (Ginnette Riquelme/Reuters)

"I don't think there is any such thing as a 'new PRI,"' said Rodrigo Aguilera, the Mexico analyst for the Economist Intelligence Unit. "There is a new generation of PRI members, but they don't represent any fundamentally different outlook."

Pena Nieto has 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.