The outfit called Golos (Voice) said ahead of Sunday's parliamentary elections it has never been under so much state pressure in a decade of documenting electoral fraud.
"It turns out that Putin is afraid of us, although I am surprised myself," Golos member Arkady Lyubarev told AFP Friday at the group's small, cluttered offices tucked away on a side street in central Moscow.
"The authorities' approval ratings are taking a beating and they're afraid that they will not receive the results they are hoping for," he said.
In September, Golos set up a "Map of violations" website (www.kartanarusheniy.ru) documenting alleged electoral fraud in the run-up to the legislative polls, and inviting Russians to contribute.
The website now features nearly 5,000 instances of alleged violations, including attempts by regional officials to pressure Russians into voting for Putin's United Russia party, often on pain of being fired. The website has been viewed more than 860,000 times since September.
The project is starting to raise questions about the legitimacy of the polls and has incurred the wrath of the paramount Russian strongman.
Speaking at a United Russia congress at the weekend, Putin lashed out at Western attempts to "influence the course of the election campaign" through Russian NGOs, warning this was "money thrown to the wind."
Icily, he added that Judas -- the disciple who betrayed Jesus -- was not the most respected biblical figure among Russians.
Golos says it does accept Western grants but only because Russian businesses are scared of supporting it and the government favours servile organisations for funding.
Following Putin's address, the small observer association with a network of several thousand volunteers across Russia, has felt the full force of the state machine.
After three members of parliament asked the authorities to probe Golos, a prosecutor showed up at the observers' offices, saying they broke election laws and ordering them to show up for a court hearing Friday.
Later Friday, the state-controlled NTV television is scheduled to broadcast a prime time documentary about Golos which the organisation has dismissed as a smear -- a tactic normally reserved for top Kremlin enemies.
The pressure comes despite calls by prominent rights campaigners and even the head of the Kremlin's own human rights council, Mikhail Fedotov, to stop the persecution.
Golos says it has never before received so much support from ordinary Russians.
"When the pressure started this week, our phones have gone red hot. People call and simply tell us: 'Please, I want to be an observer. We've never before seen people being so active during election campaigns," said spokeswoman Olga Novosad.
Golos says the FSB state security service, the successor to the feared Soviet-era KGB, routinely harasses the group's regional coordinators.
It fears officials may ramp up the pressure and even force the group to close its doors, but insists that backing down is not an option.
About 3,000 Golos observers will monitor polling stations countrywide on election day.
"These are the last elections that are going to be so blatantly falsified," said Novosad. "Something should happen in the country so that such elections can never be repeated."