Russian independent voting monitor Golos says a referendum enabling President Vladimir Putin to stay in power for the next 16 years was rigged.
Golos noted "multiple voting, ballot stuffing" and "violating voter secrecy" in an assessment of the vote.
Tens of millions of voters were not able to cast ballots at neutral venues, it said. "A significant portion of the votes were collected by voting directly at enterprises and institutions, de facto under the control of their leadership."
The results themselves were "falsified," Golos added.
Independent vote monitors under the auspices of the Organisation for Security and Cooperation in Europe (OSCE) were not invited by the Russian government to observe the voting.
The OSCE's Office for Democratic Institutions and Human Rights has traditionally provided independent monitoring of Russian presidential elections and other votes, including a 2003 referendum that affirmed Chechnya as a part of Russia.
Russia's federal electoral authority reported that more than three-quarters of the voters, nearly 78 per cent, were in favour of amending the constitution to enable Putin to run for re-election twice more.
The voting for the referendum had been conducted over the course of a week at polling stations to maintain precautions to prevent the spread of the coronavirus.
Critics had alleged that lengthening the voting process could facilitate violations.
Golos said it appeared abnormal that the vast majority of the votes had been cast before the main day of voting, Wednesday, which Putin had declared a one-off holiday to encourage turnout.
"The turnout at the early voting turned out to be anomalous ... four-fifths of the number of citizens listed as having participated in the voting did so before July 1 [Wednesday]," Golos said.
Golos, arguably Russia's most prominent independent vote monitoring organisation, has been declared by the Russian government as a "foreign agent" on the allegation that the organisation received funding from abroad. Golos has adamantly protested the label as unjustified.
Beyond giving Putin the potential to prolong his rule until he is 83 years old, the amendments boost the authority of the government's legislative branch, making it easier for the national parliament, controlled by the Putin-loyal United Russia party, to remove judges.
The amendments also clamp down on the political opposition by stipulating that presidential candidates must have lived in the country for the previous 25 years.
That essentially rules out prominent politicians Alexei Navalny, who attended Yale University in the United States a decade ago, and Mikhail Khodorkovsky, who currently lives abroad.
Navalny, who has led several Russian protest movements and is arguably Putin's fiercest challenger, has rejected the amendments and alleged that the voting results were falsified.
"A huge number of people are frustrated: Everyone was voting 'no,' and the result is a solid 'yes,'" Navalny said in a statement on Twitter. In a separate video address to his supporters, he said: "We will never know the true result."
Australian Associated Press