Members of Russia's only independent election monitoring group have told the Guardian they are being harassed and intimidated by state security officials in the runup to Sunday's parliamentary elections.

The group, Golos, was found guilty on Friday of breaking Russian electoral law, part of an apparent campaign to discredit it. The case came as the state-run television channel NTV aired a report that claimed to show the group is part of a US-funded plot to disrupt the vote on Sunday, where the ruling United Russia party is expected to make its poorest showing ever amid increasing voter discontent.

Golos's troubles began after Vladimir Putin, the prime minister, made what some employees believe to be a veiled threat during a speech in which he officially accepted the party's nomination for president. Putin is widely expected to win the 4 March vote, despite increasing disillusionment with his regime. While opposition groups and NGOs often complain of pressure from the state, they say this year the tactics appear to have been stepped up, in a sign of nervousness about the elections.

Speaking at a United Russia congress last weekend, Putin said: "We know that representatives of some countries meet with those whom they pay money – so-called grant recipients – and give them instructions and guidance for the 'work' they need to do to influence the election campaign in our country."

Golos admits it receives grant money from the west, including the United States, but says it has never hidden the fact. The group gathers complaints of election violations and publishes a "map of violations" on its website. It has so far recorded nearly 5,000 in this election cycle, most involving United Russia.

The group was fined 30,000 roubles (£620) after a Moscow court said the map violated Russian electoral law that forbids the publishing of polls within five days of the election. A letter sent to Golos by prosecutors on Thursday also argued that the group was engaged in "spreading rumours under the disguise of reliable facts in an attempt to blacken the party and some of its members".

The Organisation for Security and Co-operation in Europe, which has sent 340 election monitors to Russia, said it was "closely monitoring the situation regarding Golos. Golos has proven to be a valuable and important source of objective information to our research team."

The situation appears to be particularly tough in Russia's further-flung regions. Andrei Morgunov, regional Golos chief for the Nizhny Novgorod region, said several volunteers had been called in for questioning by various departments, including the police, the federal security service and the department for sport and youth.

The pressure appears to be working. Natalya Valkova, a 24-year-old medical student, was one of those called in. She was summoned by telephone to her university, where she was met by two men from an unspecified department. "They gave me 'friendly advice' to learn more about this Golos organisation, that I should make the right choice," she said. Valkova has since quit the group. "I'm not scared, but I think if people give me advice it doesn't come from an empty place."

"We're under colossal pressure," said Morgunov, who also claims a car has been following him since Thursday. "We're far from Moscow – they can do whatever they want."

Members of the Other Russia, an umbrella opposition organisation, also complained on Friday that several of their activists had been detained after having their flats searched.

News of the increased pressure came as President Dmitry Medvedev made a last-minute push to boost support for United Russia, which is expected to lose its constitutional majority in the Duma, or lower house of parliament. Russia's political parties have enjoyed "free and equal competition" ahead of the vote, he said.