President Viktor Yanukovych said in an address on Saturday night that he condemned "the actions that led to the forceful confrontation and the suffering of people."
He added that he was "outraged" by the violence.
Police moved in at 0430 a.m. in Kiev after 10,000 anti-government demonstrators converged on a square outside Mikhailovsky Zlatoverkhy Cathedral.
The crackdown came a day after U.S. Ambassador to Ukraine Geoffrey Pyatt warned about serious consequences if force was used against the protesters.
Witnesses say tear gas and truncheons were used.
It was horrible. We were holding a peaceful demonstration and they attacked us. They threw us away like garbage.