MEXICO CITY (AP) -- Enrique Pena Nieto took 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 congressional chamber where he swore to protect the constitution and laws of the land. The chamber erupted in cheers after a morning of violent protests, with congress members chanting "President!" and "Mexico!" Outside, protesters opposed to the new president clashed with tear gas-wielding police, but had mostly dispersed before he arrived. 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." "One word sums up Dec. 1: The restoration. The return to the past," said Congressman Ricardo Monreal of the Citizens Movement party. 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. Before Pena Nieto's motorcade arrived later that 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." 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. 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.