Latvia's parliament votes for a new president on Wednesday as the small Baltic NATO and eurozone member weighs security concerns amid heightened tensions with Soviet-era master Russia.

Four candidates are in the race, but given Latvia's indirect presidential voting system and its fractious 100-member parliament, it is unclear whether anyone will win the required 51 votes to replace centrist President Andris Berzins.

Berzins took office in 2011 and is not standing for re-election.

The vagaries of the indirect poll and its secret ballot have prompted thus far fruitless calls for reforms -- including allowing voters to choose their head of state directly.

"What goes on in parliament on election day is very much a 'black box'," University of Latvia political scientist Ivars Ijabs told AFP.

"Right now, in Latvia no one knows who's going to be the next president. It means we follow it like a football game."

The Latvian president is commander in chief of the armed forces, nominates the prime minister and has the right to propose and return legislation to parliament.

The frontrunners are Defence Minister Raimonds Vejonis of the populist Greens and Farmers Alliance party and Egils Levits, a judge at the European Court of Justice nominated by the right-wing National Alliance.

Vejonis is touting his national security credentials, while Levits says his "priorities are reform of the legal system, security and social issues."

Having shed Soviet rule in 1991, Riga has made national security a priority since Russia's annexation of the Crimean peninsula from Ukraine last year.

- 'If I had a vote' -

Also running are former basketball star and banker Martins Bondars of the small, business-friendly Regional Alliance party, and veteran parliamentarian Sergejs Dolgopolovs of the Harmony party -- who is backed mostly by Latvia's large Russian minority.

Harmony topped the country's October 2014 parliamentary election, but has been shunned by rival parties over its pro-Kremlin stance.

"If I had a vote, it would be for Martins Bondars. He would project a positive image of Latvia and he has ties with the US, which is important," Latvian voter Ilze Alksne told AFP in Riga, referring to the candidate's Harvard University master's degree in public administration.

Ethnic Russian voter Sergejs Borisovs told AFP he had no time for Dolgopolovs.

"I live near the (Russian) border. Security is the main thing, so I would be happier with Vejonis or maybe even Levits," he said.

All four candidates will contest the first vote, with the person who draws the least support dropping out. The process is repeated with three candidates, then just two.

But with MPs able to abstain and backroom deals often critical, a result is far from guaranteed Wednesday.

Should there be no winner, the whole process will recommence around 10 days later with new nominees.