The EU says it is putting on hold work on a key trade agreement with Ukraine, saying the government's arguments have "no grounds in reality". Protesters have taken to the streets in support of the agreement, which was rejected by the government.
Boxing champion Vitaly Klitschko told the crowd of anti-government demonstrators in Kiev that their protests must remain peaceful.
"We do not want to be kept quiet by a policeman's truncheon," he said.
Mr Klitschko, the leader of the opposition Udar (Punch) party, is increasingly seen as a national leader-in-waiting.
Addressing the anti-government protests in Ukraine's capital of Kiev, he demanded the release of political prisoners, the resignation of the government and early elections.
"This is a decisive moment when all Ukrainians have gathered here because they don't want to live in a country where corruption rules and where there is no justice," the leader of the opposition Udar (Punch) party said.
There is an urgent need for de-escalation in Ukraine as President Yanukovich tries to calm a country bitterly divided over years of history.
Ukraine's political and economic landscape is stuck between old Soviet masters in Russia and the democratising ideals offered by the EU.