The losing candidate in Peru's election, Ollanta Humala, has vowed to lead the opposition against the President-elect, Alan Garcia.
Mr Humala said his alliance was Peru's "primary political force"
Mr Humala said he would form "the principal opposition bloc".
With 98.7% of the votes counted in Sunday's election, former President Garcia polled 52.5% of ballots compared to Mr Humala's 47.5%.
Mr Humala's nationalist alliance won more seats than any other party in April's election to congress.
Mr Humala ruled out any form of political marriage with Mr Garcia's Apra party.
He pledged that his alliance would not allow Mr Garcia to repeat the errors of his past presidency which was marked by rampant inflation and a rebel insurgency.
"I don't have any confidence at all in Alan Garcia. He ran one of the worst governments in the history of Peru," Mr Humala said.
He said his alliance - that won 45 seats in the country's 120-member congress compared to Apra's 35 - would be the main opposition bloc.
"Look at the political map. We are the primary political force," Mr Humala said.
After the presidential campaign that polarised Peru, some analysts have suggested Mr Humala might try and start a populist revolution.
Prudent fiscal management
Slash government spending
Encourage foreign investment
Ensure foreign companies pay more taxes
Tough on crime
Wants free trade agreement with US revised
They have said Mr Humala might try to build on the base of support he has among Peru's poor and a growing disenchantment with free market policies.
During the campaign he had the support of Venezuelan President Hugo Chavez, and many feared his election would further Mr Chavez's nationalist sphere of influence in Latin America, the BBC's Hannah Hennessy in Lima reports.
But Mr Humala said he had not spoken with Mr Chavez since the elections and insisted he would lead a democratic opposition.
However, he refused to rule out street protests if, as he said, the political reality demanded it.