Conservative evangelical Christian Fabricio Alvarado Munoz has an effective lead of almost 14 percentage points over ruling party hopeful Carlos Alvarado Quesada in the race to be Costa Rica's next president, an opinion poll showed on Friday.

Alvarado Munoz, a 43-year-old religious singer and former journalist who belongs to the National Restoration Party, shot to prominence after condemning a court ruling that urged Costa Rica to grant civil marriage rights to same-sex couples.

The survey by polling firm OPol Consultores conducted from March 12-16 gave Alvarado Munoz 56.9 percent effective support ahead of a second round run-off on April 1, once undecided voters and those planning to skip the vote were stripped out.

That left his younger rival Alvarado Quesada, a former minister in the centre-left Citizens' Action Party of President Luis Guillermo Solis, on 43.1 percent. Alvarado Quesada is a supporter of gay marriage.

Over one third of the electorate was undecided or planned not to vote, the poll for online newspaper El Mundo showed.

If those Costa Ricans were factored in, Alvarado Munoz had the backing of 36.5 percent of voters, and Alvarado Quesada 27.7 percent support, the polling firm said.

The poll had a margin of error of 1.7 percentage points, and was based on interviews with 3,300 people nationwide.

Surveys by OPol have shown a clearer advantage in the race for Alvarado Munoz than others. A University of Costa Rica poll published on March 6 gave Alvarado Quesada a slight lead.

Alvarado Munoz was elected to the national assembly in 2014 as the only federal deputy representing the Christian-backed National Restoration Party.

His rise on a ticket opposing gay marriage was helped by the decline of a two-party system going back decades in a country that has long been considered one of Latin America's most stable.