The 18 February 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 (see background).
"This is a vote about the foundations of the Latvian state," Prime Minister Valdis Dombrovskis told reporters after he voted against the measure.
"Against", was the message in giant-sized letters etched in the snow on the frozen river Daugava in Riga.
Amongst the Russian-speaking population, the vote was 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 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 more than 187,000 signatures in the vote's support, forcing the Latvian government to organise a nationwide referendum.