Several thousand Paraguayans held a protest rally in the country's capital last night, claiming there was electoral fraud in the primary to pick the ruling party's candidate for the April 20th presidential election.
The Colorado Party, which has governed for more than 60 years, is split after the primary vote last month ended in a virtual tie.
Former vice president Luis Castiglioni organised the rally after accusing electoral judges of swinging the vote in favour of his rival, Blanca Ovelar.
The impoverished, landlocked South American country has a long history of corrupt politics, and leading opposition candidate Fernando Lugo - a former Roman Catholic bishop - endorsed the protest but did not participate.
Mr Castiglioni accused the government of tampering with polling station documents in 11 of the country's 17 provinces.
"They can falsify the voting documents but when we govern, things are going to change," the 45-year-old civil engineer and businessman said.
While he spoke, a small plane flying overhead dumped pro-Ovelar pamphlets, angering the protesters who were shouting, "Stop the fraud!"
Mr Ovelar was shown edging out Mr Castiglioni in preliminary results and throughout much of the recount, but the former vice president insists he won the contest.
Final results from the center-right Colorado Party's primaries are due to be announced on Monday, after a month of recounts and delays.