"It was as if the president took a pencil and drew the train route and how much it would cost to Toluca," Silva-Herzog said. "It was very concrete, very practical, zero ideology ... this is Pena Nieto. I think Pena Nieto is not a person who thinks in abstract terms."
Many remain to be convinced.
Before he took the oath of office, leftist congressional 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, who assumed office at a midnight ceremony at the National Palace, campaigned as the new face of the PRI, repentant and reconstructed after being voted out of the presidency in 2000.
Before his public swearing-in at mid-day, hundreds of opponents banged on tall, steel security barriers around Congress, threw stones, bottle rockets and firecrackers at police and yelled "Mexico without PRI!" Police responded by spraying tear gas from a truck and used fire extinguishers to put out flames from cocktails. One group of protesters rammed and dented the barrier with a large 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 wouldn't give his last name, saying he feared reprisals.
"He gave groceries, money and a lot more so people would vote for him," the student added, referring to allegations that the PRI gave voters gifts to encourage them to cast ballots for Pena Nieto.
Protesters trailed the new president from the Congress to the National Palace, trying to break down the barriers set up in the Zocalo, Mexico City's giant central plaza in front of the palace.
"The president is like Salinas: 'I don't see you, I don't hear you,'" said Aurelio Medina, 64, a vendor and protestor referring to PRI President Carlos Salinas de Gortari.
Lines of riot police closed down streets around the Fine Arts Palace near where Pena Nieto gave his speech. Police arrested a few protesters who were throwing rocks or pieces of wood. Windows of a Sears departmental store were smashed and its outside walls splashed with white paint.
Despite the protests, the atmosphere inside Congress during the swearing-in ceremony 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 the building so he could take the oath of office after a razor-thin, disputed victory over a leftist candidate.
Associated Press writers Adriana Gomez Licon, Michael Weissenstein, Carlos Rodriguez and Juan Diego Quesada contributed to this report.