A monitoring group set up by protesters in Russia has refused to recognise the results of the presidential election which returned Vladimir Putin to power.
The League of Voters said there had been widespread fraud and the poll was an insult to civic society in Russia.
Mr Putin, it added, won 53%, not 63.6% as reported officially. Such a result would have still brought him victory.
Vladimir Putin brushed aside the League's allegations, but appeared to make some concessions.
Mr Putin was re-elected for six years having served two previous terms as president between 2000 and 2008.
His victory has been recognised by foreign states but the Russian authorities stand accused by international observers of having skewed the election in his favour.
Police detained at least 550 protesters at rallies the day after the election but many have since been freed.
Protest leader Alexei Navalny, who was himself briefly detained in Moscow, said rallies would go on "until we win".
The international monitors, from the Organization for Security and Cooperation in Europe (OSCE) and the Council of Europe, urged Russia to carry out a thorough investigation into the claims of fraud.
In a statement on the OSCE website,the monitors said that while all candidates had been able to campaign freely, there had been "serious problems" from the start.
"The point of elections is that the outcome should be uncertain," said Tonino Picula, co-ordinator of the OSCE mission.
A statement from the League of Voters said it considered the election to have been unfair and dishonest, and the results falsified.
"On 4 March, civic society in Russia was dealt an insult and the institutions of the Russian presidency, the Russian electoral system and the Russian Federation's state authorities as a whole were discredited," it said.
"The count was accompanied by systematic falsification."
Among the League's findings was that the number of voters who had voted in special circumstances, using absentee ballots for instance, had risen by three million.
Abuse of absentee ballots has been one of the main accusations of fraud.
The League of Voters was set up in December in response to alleged widespread fraud at the parliamentary election that month.
An umbrella organisation of activist groups, it is fronted by celebrities such as rock singer Yuri Shevchuk and detective novelist Boris Akunin.
Leonid Ivlev, deputy head of the Russian central electoral commission, responded to the League's accusations by saying it lacked sufficient data to draw such conclusions.
"The League of Voters is carrying out some task it was set earlier," he added.
Mr Putin told Russian news agency Interfax that the League's complaints were "nothing new".
"First they acknowledged that your humble servant won more than 50% and now they've had a think, picked their noses and decided that, no, that is too much for them," he told Interfax in Moscow.
Mr Putin said that if violations had taken place, they had been too insignificant to influence the final results.
The president-elect appeared to make several concessions to his critics, saying he agreed the criminal conviction of former tycoon Mikhail Khodorkovsky should be reviewed, and suggesting that opposition candidate Mikhail Prokhorov might play a role in government.
He defended the actions of Russian police who broke up unapproved election protests on Monday and appeared to support the prosecution of members of a feminist punk group, Pussy Riot, for playing a "blasphemous" song inside a Moscow cathedral.
"I hope it doesn't happen again," he said.