The confrontation occurred as protesters attempted to block a police van carrying three men who had been convicted of attempting to blow up a statue of Vladimir Lenin, RIA Novosti reported Saturday.
Yury Lutsenko, an opposition leader and former police chief, was beaten as he intervened to stop the fighting, said his wife, Irina, the report said.
He received head injuries and is in intensive care, she said.
Lutsenko is one of the leaders of the "Euromaidan" camp in Kiev set up in protest of President Viktor Yanukovych's reversal on a plan to sign a trade deal with the European Union, Voice of America reported. Instead, the president established stronger ties with Russia, which has promised to buy $15 billion of Ukrainian debt and discount the price of Russian gas.