The voting to elect members of the State Duma is Russia's first parliamentary election since the Kremlin launched a full-scale invasion of Ukraine on February 24, 2022.
The three-day election, which began on Friday and ends on Sunday, features virtually no opposition and is all but certain to secure the Kremlin's political dominance.
Yabloko said that officials at the polling station accused a party election observer of attempting to "disrupt the elections" and called the police, which "used force" against him.
It was unclear whether he received prompt medical attention after falling unconscious, the party said in a statement on Telegram, adding that he was taken to a police station and charged with disobeying police orders.
Russian officials did not immediately comment on the incident.
Yabloko, the only registered political entity that dared to criticise Russia's war with Ukraine, was removed from the ballot last month by Russia's Supreme Court, and only has a few dozen candidates in the single-constituency races.
Additionally, some anti-war politicians have been imprisoned or forced to flee abroad before the campaign.
Yabloko and the Communist Party, which has criticised some aspects of government policy but carefully avoided any criticism of Russian President Vladimir Putin, said that many of their election observers were turned away from polling stations.
The two parties also reported various violations, including sealed ballot boxes being opened, the refusal to issue paper ballots and provide turnout data for verification, and doors to polling stations not being secured overnight.
On Friday, Yabloko's leader appealed to Ella Pamfilova, head of the Central Election Commission, over what the party said were numerous denials of entry to its observers and pressure on them.
"The scale of what's happening does not allow us to view this as a misunderstanding," Yabloko chairman Nikolai Rybakov said.
Russia did not invite observers from the Organisation for Security and Cooperation in Europe to monitor the election, with the Russian Foreign Ministry accusing the body of "double standards" for its criticism of Russia.
It said that it had invited other international organisations as well as observers from OSCE member states, in its place.
OSCE parliamentary assembly president Pere Joan Pons said that the decision was a breach of Russia's commitments as a member state of the organisation.
"This persistent rejection of independent scrutiny follows a deeply worrying pattern of disregard for the principles of transparency, democratic accountability and the rule of law," he said.
Russia's Central Election Commission said that more than 380,000 electoral observers had been registered before the vote, including representatives from 11 political parties.
Of those registered, 97,040 observers were linked to the United Russia party, with the next largest group, 42,743 observers, representing the Communist Party.
It also said that 1126 international election observers and "electoral experts" have been registered from 133 countries and 13 international organisations.
Russia also tightened restrictions on election observers in the lead-up to the parliamentary vote.
Under a new code of ethics, observers are banned from making statements "that could be construed as undermining confidence in the electoral system".
Russian authorities spread the parliamentary election over the weekend in a bid to boost turnout among the 111 million eligible voters, and they allowed electronic balloting.
For the first time, the parliamentary election includes residents of four Ukrainian regions that Russia annexed after the war began.
