Three members of the feminist movement Femen and their photographer were abducted in Kiev Saturday before a planned protest against Russian President Vladimir Putin's visit to Ukraine, the group said.
The alleged attack on a group known for their bare-breasted protests took place as Ukrainian President Viktor Yanukovych welcomed his Russian counterpart, Serbian leader Tomislav Nicolic and Moldovan President Nicolae Timofti for politically sensitive religious festivities in the capital Kiev.
"When three Femen activists and their photographer tried to leave for the site of the planned event, they were attacked by a group of unknown assailants and beaten up," the group's leader Anna Hutsol told AFP by phone.
After that the four were bundled into a car which left "in an unknown direction" she said, adding that the protest was subsequently cancelled.
However, police told AFP they were unaware of the incident.
Hutsol claimed that she had also come under attack earlier on Saturday.
Entering her apartment building after walking her dog in the morning, she said she was hit by an unidentified man who grabbed her pet and ran away.
"An unknown man jumped at me from upstairs, hit me in the face, grabbed my dog and fled," Hutsol told AFP, saying she had noticed being followed the day before.
"I ran after him but only saw that he had got into a car and left with my dog."
"I do not care who he is -- a member of the SBU (Ukrainian Security Service) -- or a hired bandit," she said.
She said the attack was an act of intimidation during the high-profile festivities to celebrate the 1025th anniversary of the arrival of Christianity in Ukraine and Russia.
Despite the attack, Hutsol said the group would still try to stage a protest against Putin's two-day visit to Ukraine.
"All this intimidation and violence will not stop us," she said.
Saturday's attacks come after a leading male activist with the group, Viktor Svyatskiy, was brutally beaten up last Wednesday.
The group blamed the attack on the Ukrainian and Russian special services.
Svyatskiy was hospitalised with severe bruising to his face, a suspected broken jaw, broken teeth and loss of blood.
Femen's female activists have become well-known in Ukraine and abroad for baring their breasts to protest discrimination against women and other rights violations.
The group has said Putin and the head of the Russian Orthodox Church, Patriarch Kirill who is accompanying him to Ukraine, are its "longstanding enemies" and described Saturday's celebrations as "the holy gathering of post-Soviet dictators".