With results from 53 percent of polling stations counted, the ruling party candidate had 50.87 percent of the vote, 13 points more than needed to win the election outright in the first round.

Fernandez's nearest rival, centrist economist Alvaro Ramos, was at 31.63 percent.

Costa Rica, long seen as an oasis of stability and democracy in Central America, has become a logistics hub for Mexican and Colombian drug cartels.

Drug trafficking has spilt over into local communities, fuelling turf wars that have caused the murder rate to jump 50 percent in the past six years.

Fernandez cites iron-fisted Salvadoran President Nayib Bukele, who has locked up thousands of suspected gang members without charge, as an inspiration on how to tamp down crime.

A victory for the 39-year-old political scientist would confirm a rightward lurch in Latin America, where conservatives have come to power in Chile, Bolivia and Honduras in recent months.

Bukele rushed to congratulate "president-elect" Fernandez on Sunday, wishing her "every success."

- Attacks on the judiciary -

Fernandez is the protege of popular outgoing conservative President Rodrigo Chaves, whom she served as planning minister and chief of staff.

Chaves deflected criticism for rising violence on his watch by placing the blame on what he sees as an overly-permissive judiciary.

Jessica Salgado, 27, said she voted for Fernandez as the continuity candidate, because she felt the government was on the right track, even if violence is on the up.

"The violence exploded because they (the government) are going after the ringleaders, it's like dragging rats out of the sewer," Salgado told AFP.

Costa Ricans also voted for members of the 57-seat Legislative Assembly on Sunday.

Fernandez is hoping to win a big enough parliamentary majority to change the constitution and overhaul the judiciary.

Her detractors fear she will try to change the charter to allow her mentor Chaves to return as president after her four-year mandate ends.

Under the current constitution, he is barred from seeking re-election until he has been out of power for eight years.

As he voted on Sunday, former president Oscar Arias, winner of the 1978 Nobel Peace Prize, warned that the "survival of democracy" was at stake.

"The first thing dictators want to do is to reform the Constitution to stay in power," he said, alluding to Chaves.

Ramos, the candidate of the National Liberation Party (PLN) who sought to rally opposition to Fernandez, warned that "modern dictatorships don't always arrive with tanks."

Fernandez insists she is committed to upholding Costa Rica's democratic tradition.

- Cocaine-smuggling hub -

The drug trade has sucked in the high-density "precarios" (informal settlements) of cities such as the capital San Jose, where shootouts between rival drug gangs are increasingly frequent.

Fernandez has vowed to complete construction of a maximum-security prison modelled on Bukele's brutal CECOT penitentiary.

She has also promised to stiffen prison sentences and to impose a Bukele-style state of emergency in areas worst hit by crime.

Bukele is a hero for many in Latin America, credited with restoring security to a nation traumatized by crime.