Enrique Pena Nieto has been sworn in as Mexico's new president amid violent protests outside the congress.
Dozens of people were injured on Saturday as police clashed with protesters who claimed that Pena Nieto's election was fraudulent.
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The demonstrators were also opposed to the "war on drugs" and subsequent militarisation of the country as well as labour reforms that they feel favour foreign firms and take away jobs from ordinary Mexicans.
Police fired tear gas as hundreds of protesters threw stones and bottles and shouted "Mexico without PRI" - Pena Nieto's Institutional Revolutionary Party.
Activists alleged that dozens had been arrested by the police during the protests.
Nieto formally took office shortly after midnight, when outgoing president Felipe Calderon transferred power to his successor.
Nieto said he would emphasise security for Mexicans and their families, and would work to ensure that roads and cities were again "peaceful areas where Mexicans can travel safely without fear of loss of their liberty or life".
Outside of the palace, many did not buy Pena Nieto's pledges.
Carmen Lopez, a teacher, was angry about the PRI returning to power. "He represents the PRI that for more than seven decades governed Mexico, and the only thing they left behind was poverty, underdevelopment, and above all a lot of corruption," she said.
'Direct line to old PRI'
The party's reputation was marred by corruption, authoritarianism and frequent allegations of vote-rigging during its 71 years of unbroken rule that ended in 2000.
Despite losing the presidency, the party held on to most of the country's 32 governorships.
Those statehouses served as a launching pad to reclaim the presidency when Pena Nieto won the election earlier this year, defeating leftist rival Andres Manuel Lopez Obrador.
The new president's cabinet represented the influence of what many in Mexico call "the governors' club".
Pena Nieto himself served as governor of Mexico state, which adjoins Mexico City, and almost a third of his new cabinet members have been governors.
"There is direct line to the old PRI," said Rodrigo Aguilera, the Mexico analyst for the Economist Intelligence Unit.
"I don't think there is any such thing as a 'new PRI,"' Aguilera added. "There is a new generation of PRI members, but they don't represent any fundamentally different outlook. That doesn't mean that this cabinet can't do its job well."