Russia said on Sunday Latvia had "breached its international obligations" after the Baltic state declined to allow a Russian observer mission at a referendum which rejected a proposal for Russian to be made the country's second official language.
Latvia's decision caused "bewilderment," Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement.
"What we are talking about here is that Riga ignored its international obligations. We demand an explanation," Lukashevich said.
With nearly all the votes counted, 75 percent of votes cast in Saturday's referendum were against the proposal, Latvia's election officials said.
Ethnic Russian make up about one-third of Latvia's 2.1 million population.
Lukashevich said the ballot, called by the Russian-speakers' movement, Native Tongue, "did not fully reflect the mood in the country."
He said 319,000 Russian speakers remain without citizenship and could not vote in Saturday's referendum.
"We hope the voice of Latvia's Russian-speaking population will be heard," Lukashevich said.
Latvian President Andris Berzins earlier said there was "no need" for a second language, and that the government funded minority-language schools.
"Whoever wants, can use their language at home or in school," Berzins was quoted by the BBC as saying.