Latvians were expected to reject Russian as an official language on Saturday in a controversial referendum that has heightened ethnic tensions in the former Soviet nation and triggered renewed criticism from old imperial master Moscow.

The referendum was initiated by Latvia's pro-Russian lobby, which says the Baltic state's large Russian-speaking minority has been shut out of political life since Latvia broke free from the Soviet Union in 1991.

Latvian nationalists see the vote as a Kremlin-backed attempt to weaken the country's sovereignty in order to push it back into Russia's sphere of influence.

To call things as they are, this referendum is a test for traitors to the state, popular theatre director Alvis Hermanis said on public television this week, expressing the view of many ethnic Latvians about those who back Russian as an official language.

Latvia regained its independence in 1991 after 50 years of what it sees as Soviet occupation. Post-independence laws were aimed at weeding out Russian influence and boosting the status of Latvian language and culture.

This is a vote about the foundations of the Latvian state, Prime Minister Valdis Dombrovskis told reporters after he voted against the measure. He said he expected a high turnout and a decisive vote against adopting Russian as a second official language.

Many Russian speakers settled in Latvia during the Soviet period and make up about a third of the two million population. They are viewed by some Latvians as illegal occupiers.

Against, was the message in giant-sized letters etched in the snow on the frozen river Daugava in capital city Riga.

The pro-Russian lobby is almost certain to lose, given that the majority ethnic Latvian population will vote against. About 770,000 ballots in favour are needed, higher than the number of Russians with the right to vote.

Among the Russian-speaking population, the vote is seen as a way to protest against measures they say discriminate against them, including a requirement to take language and history tests to become Latvian citizens.

Russian speakers who refused this naturalisation process and were left as non-citizens, with no right to vote or take jobs in the public sector. They argue they pay taxes like everyone else, have lived in Latvia for decades and say Latvians should forget the wrongs of the Soviet period.

Do not lose your chance to show your attitude to what's been happening in Latvia for the last 20 years, said Russian-language newspaper Vesti Sevodnya in a front page headline.

The referendum is a stage in the fight of Latvia's Russian residents for their rights, said vote organiser Vladimir Linderman, who speaks fluent Latvian but has yet to naturalise as a citizen and could not vote.

His For the Mother Tongue group collected over 187,000 signatures in the vote's support, forcing the Latvian government to organise a nationwide referendum.

Though Latvia's ties with neighbour Russia have improved in recent years from their immediate post-Soviet low, Moscow has slammed what it sees as discrimination of Russian speakers and has described the vote as a cry for help from ethnic Russians.

Inter-ethnic issues were not addressed and Harmony Centre (the main ethnic Russian political party) was not allowed to join the government, said Ambassador Alexander Veshnyakov in a recent interview in Vesti Sevodnya.

The referendum is a manifestation of dissatisfaction with the situation, he added.

(Reporting By Aleks Tapinsh,; Editing by Rosalind Russell)