San José: Costa Rica's government on Tuesday said it had uncovered a plot to assassinate President Rodrigo Chaves on the eve of national elections in which his right-wing party is tipped for victory.

Jorge Torres, head of the Central American nation's Directorate of Intelligence and National Security, cited a "confidential source" as informing the agency that an assassin had been paid to kill Chaves.

Attorney General Carlo Diaz told reporters a female suspect was under investigation and described her as "quite active on social media" but gave no other details.

He said there was no evidence of a link to the presidential and parliamentary elections set for February 1.

The popular Chaves, constitutionally barred from seeking a second consecutive term, has backed one of his former ministers, Laura Fernandez, to succeed him.

Fernandez is riding high in polls with a campaign to crack down hard on drug traffickers, blamed for a surge in violence in the picture-postcard Caribbean country.

Opposition groups have warned against possible interference in the election from the iron-fisted president of nearby El Salvador, Nayib Bukele, who started a state visit to Costa Rica Tuesday.

He is scheduled to lay the cornerstone Wednesday of a new mega-prison modelled on El Salvador's brutal Terrorism Confinement Center (CECOT).

Thousands of young men are being held without charge in CECOT as part of Bukele's war on criminal gangs.

Several Venezuelans, who were deported from the United States to the facility last year, claimed upon their release that they were tortured. (AFP)