ASUNCION (Reuters) - Supporters of a right-wing candidate who came third in Paraguay's presidential election clashed with police outside the electoral court on Monday, amid complaints of fraud in a vote that the ruling Colorado Party won comfortably.

Police put up fences around the court's headquarters and fired rubber bullets at young protesters who were throwing stones, authorities said, after hundreds of supporters of Paraguayo Cubas gathered.Elsewhere, demonstrators blocked roads with burning tires and destroying billboards with the photo of President-elect Santiago Pena, a 44-year-old economist who won 43% of the vote on Sunday compared with 27% for runner-up Efrain Alegre.

Cubas, who surprised observers by winning nearly 23% of the vote, called in a post on Instagram for a recount and asked his supporters to protest.

"We are not satisfied. The elections were stolen from us. It's that simple," Yolanda Paredes, a senator-elect and the wife of Cubas, told reporters.

Cubas was due to travel on Tuesday from his stronghold in Ciudad del Este, on the border with Brazil, to the capital, Asuncion.

Runner-up Alegre said on Twitter that he too was calling for the electoral court to do a recount and for an international audit of the computer programs used in electronic ballot boxes.

Interior Minister Federico Gonzalez called for "sanity".

"A procedure is being followed and that must be respected," he said, referring to the work of the electoral court.

(Reporting by Daniela Desantis; Writing by Brendan O'Boyle; Editing by Robert Birsel)