The rally was spurred by legislation rushed through parliament last week, which the opposition says will lead to a police state.
The legislation prohibits unauthorised installation of tents or stages and the use of loud-speakers in public places. "Mass disorder", the wearing of helmets and face masks and dissemination of "libellous information about the country's leaders are also now illegal.
Yesterday's clashes erupted as demonstrators, including supporters of jailed opposition leader Yulia Tymoshenko, encountered police cordons as they headed for parliament.
They attempted to overturn a bus commandeered by the police, and stun grenades were fired to clear the area.
Protestors denounced last Thursday's vote in parliament by supporters of President Viktor Yanukovich.
"Ukraine is united as never before in its struggle against those in power today, in its determination not to allow a dictatorship," declared boxer-turned politician Vitaly Klitschko, the strongest potential challenger for the presidency.
Mr Yanukovich triggered the crisis last November when he ditched a trade deal with the EU in favour of closer ties with Russia.