An Odessa regional police spokesman, Vladimir Shalivenko, told Agence France-Presse that the case was being investigated as an assault involving “minor bodily harm.”

Ms. Shevchenko said that Mr. Svatsky and Ms. Hutsol had been treated at a hospital and were recovering, but that at the time of the attack on Saturday had not yet healed from previous assaults in Kiev.

Last month, Mr. Svatsky was beaten savagely near Femen’s office in Kiev and spent two weeks in a hospital. Then, Ms. Hutsol was attacked outside her apartment and her dog was stolen, and, on the same day, Ms. Shevchenko, two other Femen members and a photojournalist were beaten and detained overnight by the Ukrainian authorities.

Those attacks coincided with a visit to Kiev by President Vladimir V. Putin of Russia and Patriarch Kirill I, the leader of the Russian Orthodox Church, for festivities related to the anniversary of the church’s founding.

Ms. Shevchenko said the group had initially believed that the attacks were instigated by the Russian government to prevent a repeat of a situation early this year in Hanover, Germany, in which Ms. Shevchenko, topless, ran toward Mr. Putin, who was visiting a trade show with Chancellor Angela Merkel. Ms. Shevchenko, who had a profane slogan written on her chest accusing Mr. Putin of being a dictator, came within a few feet of the Russian leader before security officers wrestled her to the ground.

Odessa, which is known as a hub for international sex trafficking, would seem to be a logical place for a Femen protest. But it is also one of Ukraine’s most popular resort cities, and the group’s leaders said the only thing they had planned was time at the beach.

Ms. Shevchenko said that in recent days the group had let down its guard. “We were a little bit relaxed,” she said. “We thought that everything was over, and it was just guys from Russia. We went to Odessa just to have a small vacation.”