Peruvian socialist Pedro Castillo maintained a slim lead over rival Keiko Fujimori on Thursday morning with 99.998% of the votes counted and as electoral authorities met to scrutinise contested votes.

Castillo, an elementary school teacher and political novice who won widespread grassroots backing for pledges to rewrite the constitution of the world's No. 2 copper producer and redistribute wealth, had 50.2% of the vote, maintaining a 0.4 percentage point lead over right-wing Keiko Fujimori, or 71,441 votes.

Some 300,000 contested votes are being scrutinized by an electoral jury, a process that will take several days to complete and could delay the announcement of who will be the next president to take over from interim leader Francisco Sagasti at the end of July.