Protesters in Kiev clash with police.
Tensions are mounting, with President Viktor Yanukovich refusing to sign a free trade deal with the European Union.
"Ukraine is Europe", protesters were chanting.
Some 10,000 of them rallied in Kiev's Independence Square, site of the Orange Revolution in 2004.
Several people were injured, including a Reuters photographer who was hit on the head.
The clashes broke when protesters tried preventing police from removing two vans used by the supporters of the opposition.
Opposition leaders demanded the resignation of Yanukovich, accusing his government of corruption.
Speaking to supporters, former heavy weight boxing champion Vitaly Klitschko said: "The authorities must be held responsible for not signing the EU agreement, and we must make sure they no longer exist."
Yanukovich's decision to abandon the trade deal brings Ukraine closer to Russia and deals a blow to EU efforts to build closer ties with its
