MEXICO CITY -- Enrique Pena Nieto assumed the presidency of Mexico Saturday amid high hopes that his muscular, once-autocratic political party, which governed this country for most of the 20th Century, will heal a bruised, bloodied nation and rev the economy.

Pena Nieto, 46, donned the red, white and green presidential sash shortly before noon in front of a raucous Congress and turmoil in city streets.

Near Congress and in front of historic Alameda Park, young protesters opposed to Pena Nieto threw rocks and bottles at riot police, set fire to a bank branch, smashed windows of a Hilton Hotel and numerous other storefronts and ran from a cascade of tear gas. An undetermined number were injured.

The moment marked a chaotic return to power for the Institutional Revolutionary Party, or PRI, which was ousted at the ballot box in 2000 after 71 years of near absolute rule.

Pena Nieto, a telegenic former state governor, persuaded voters in July 1 elections that a revived PRI had abandoned its old, autocratic practices and would undo some of its policies of decades past, such as blocking foreign investment in the oil sector, in a bid to bring greater prosperity to Latin America’s second-largest economy.

He hit upon those same themes in his first speech as president, delivered at the ornate National Palace before hundreds of Mexican dignitaries and foreign heads of state and envoys, including Vice President Joseph Biden.

“Mexico has not achieved the advances that its people demand and deserve,” Pena Nieto said. “Insecurity and violence have robbed the peace and freedoms of diverse communities in our territory.”

He outlined programs to alleviate poverty and hunger, protect single mothers and the elderly and build new infrastructure, such as a passenger train system.

“We are a nation that grows at two speeds. There is a Mexico of progress and development. But there is also one that lives in backwardness and poverty,” he said.

He only talked obliquely about the drug-related turmoil that has wracked Mexico over the past six years. Crime gangs engaged in beheadings, mass slaughters and firefights for smuggling routes have left a horrendous toll: More than 60,000 people – perhaps far more – have died in bloodshed since late 2006.

In addition, as many as 10,000, and perhaps more, Mexicans and Central American migrants have gone missing, kidnapped by ruthless crime groups or security forces.

Pena Nieto promised to cut homicide rates in half and reduce street crimes.

Yet Pena Nieto appeared determined to de-emphasize the fight against crime and focus more intensely on economic growth, which only recently has picked up after a lackluster decade.

If Pena Nieto is successful, the impact will ripple to the United States, where six million jobs depend on exports to Mexico.

Many Mexicans will watch carefully to see if the PRI resorts to the corruption that once marked its governing style, including handing off business monopolies to loyalists.

“The government that starts today has a whiff of the old and the known,” Alberto Aziz Nassif, a sociologist, wrote Saturday in the Mexico City newspaper El Universal.

Fears that the party has not changed were palpable. Full page newspaper ads lamented the “death to democracy” in Mexico and lambasted the PRI for “buying” the July 1 election with the support of Televisa, a huge television and media conglomerate.