Mexico's new President Enrique Pena Nieto pledged Saturday to reduce the poverty and violence plaguing his country after a rowdy swearing-in ceremony marked by protests inside and outside Congress.

His inauguration brings back the Institutional Revolutionary Party (PRI), which ruled Mexico with an authoritarian hand for 71 years until it lost its first election in 2000, but the lofty ceremony was preceded by trouble.

Thousands took to the streets, chanting "Mexico without PRI," amid a tight security cordon and with tall metal barricades surrounding the Congress.

A group comprising around 500 protesters, many wearing masks, threw Molotov cocktails and objects at the police, who responded with tear gas.

More clashes erupted near the historic national palace, with protesters looting and wrecking the fronts of shops, hotels and restaurants, culminating in 92 arrests and 76 people being injured, said authorities and the Red Cross.

With a telegenic smile and TV star wife, Pena Nieto, the 46-year-old former Mexico state governor has projected an image of a changed PRI that has left its darkest days behind.

"Democracy has cemented itself and become part of our culture," he said in a speech at the national palace in front of Mexican and foreign dignitaries, including US Vice President Joe Biden.

The new Mexican leader succeeds Felipe Calderon, who handed him Latin America's second biggest economy but a relentless drug war that has killed more than 60,000 people in the last six years.

Pena Nieto presented a 13-point plan to boost growth, fight poverty and hunger, and cut a crime wave that has included street gunfights, beheadings and kidnappings that have brought fear to many parts of the country of 110 million.
“The first job of my government is to have peace in Mexico," he said. proposing a comprehensive crime prevention program and reform of the penal code in a nation where only one percent of crimes end in conviction.

"Cities, villages and roads must again be safe areas, where Mexicans can go without fear of losing their freedom or lives," he said, calling for the passage of a stalled victim's protection law.

Earlier, Pena Nieto took the oath of office in Congress, vowing to defend the constitution as supporters chanted "Mexico! Mexico!" drowning out wailing whistles of disapproval from leftist lawmakers.

The opposition mood was also defiant inside Congress, with leftist lawmakers unfurling a giant black banner with crosses and the words "Imposition accomplished, Mexico in mourning."

The second-place finisher in this year's election, leftist Andres Manuel Lopez Obrador, has refused to concede defeat, charging that the PRI bought millions of votes. But the electoral court threw out his claims.

"A spurious government has concluded and the nightmare of imposition, illegitimacy and return to the past begins," Deputy Ricardo Monreal, who was Lopez Obrador's campaign manager, told Congress.

Six years ago, leftist lawmakers took over the legislature's podium in a bid to disrupt Calderon's swearing-in ceremony. This time, PRI representatives stood by the steps in an apparent effort to prevent a repeat.

"Today is a day of happiness, hope and innovation," said Deputy Arturo Escobar of the Green Party, an ally of the PRI.

Pena Nieto wants to push through structural reforms to boost Mexico, which posted stronger growth than regional powerhouse Brazil last year.

But he faces a relentless battle against powerful drug cartels, who continue to smuggle crystal meth, cocaine and marijuana to the United States even though 25 of the 37 most wanted kingpins have been captured or killed.

Calderon deployed 50,000 troops to crack down on criminal groups such as the Sinaloa, Zetas and Gulf cartels, but analysts say the strategy backfired as the captures generated fresh street battles for control of trafficking routes.