Pedro Castillo, a rural schoolteacher, has promised to upend a quarter-century of neo-liberal government. Castillo said he would send a bill to parliament with a view to organising a referendum on replacing the constitution.
Leftist political novice is sworn in as country's fifth president in three years as it celebrates 200th anniversary of independence.
Uncertainty over the June 6 vote mounts as supporters of socialist Pedro Castillo and right-wing rival Keiko Fujimori take out rallies.
Political tensions remain high in Peru following the June 6 election in which socialist Pedro Castillo declared himself the winner. But his rival conservative Keiko Fujimori has levelled accusations of election fraud.
Under Peruvian law, an election victory for Keiko Fujimori would see graft charges suspended until after her term, but defeat would see her go to trial.
Right-wing presidential candidate Keiko Fujimori raises allegations of "irregularities" and "signs of fraud" in Sunday's run-off election as her rival, the far-left trade unionist Pedro Castillo, took a razor-thin lead in the vote count.
The statistical tie had Fujimori leading with 50.3 percent of the vote, while Castillo had 49.7 percent, within the margin of error of plus or minus 3 percentage points, according to the poll.
With nearly 90 percent of ballots counted, leftist Pedro Castillo was in the lead with 18.83 percent - much higher than predicted by opinion polls which had not even placed him in the top five.
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