As several hundred protesters threw fire bombs at police and smashed plate glass windows, Pena Nieto marked the return of the Institutional Revolutionary Party, or PRI, with a 13-point plan heavy on old-party populist handouts but with reforms designed to boost the economy and modernize the education and justice systems.
"Mexico has not achieved the advances that the population demands or deserves," Pena Nieto said in an inaugural speech unusual for its heavy emphasis on policy. "It's time for us together to break the myths and paradigms and all else that has limited our development."
Inaugural events were marred all day by protesters opposed to the return of the PRI after a 12-year hiatus.
Inside and outside the congressional chambers where he took the oath of office, his opponents called his inauguration an "imposition" of a party that ruled for 71 years using a mix of handouts, graft and rigged elections.
At least 76 people were treated for injuries, including 29 who hospitalized, as the result of clashes between protesters and tear-gas firing police, the Red Cross said. City officials said 103 people were detained, including 11 minors.
Vandals smashed windows of stores, banks and a hotel and made bonfires of furniture dragged into the streets. One downtown bank office where all the windows were broken had the words "Welcome Pena" painted across the facade in green.
Pena Nieto countered with a speech full of specifics, from creating an integrated crime prevention program to ending the patronage and buying of teacher positions that rule the public education system.
He said he will put security at the center of all policies for Mexicans and their families and will work to ensure that roads and cities are again "peaceful areas where Mexicans can travel safely without fear of loss of their liberty or life."
Mexico has suffered a spike in violence since outgoing President Felipe Calderon launched an offensive against organized crime upon taking office six years ago. Some 60,000 people have been killed by drug violence since then, according to some estimates. While officials first said most of the victims were involved in organized crime, the killings and kidnapping spread to innocent civilians as drug gangs came to rule entire towns and even parts of some states.
Pena Nieto turned to his usual style of result-oriented governing with the list, having started his term as governor of Mexico State with 608 projects that he promised to complete.
The tone of his speech was conciliatory, an attempt to alleviate fears about a return to the PRI's autocratic past.
"I will respect every voice," he said. "I will run an open government that speaks with honesty, seeks opinion, listens to its citizens ... I will be a president who is close to the people."
Many of his proposals harkened back to the old populist PRI, promising pensions for the elderly, life insurance for single mothers to support their children through college, a program to end hunger and a new system of passenger trains.
Political analyst Jesus Silva-Herzog Marquez marveled at the specificity.
"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.