Ukrainian President Viktor Yanukovych has said he is "deeply outraged" by events at opposition protests in Kiev.
He was speaking after police violently dispersed an opposition camp on Saturday. Protesters were set to return to the capital again on Sunday.
Opposition parties are angry Mr Yanukovych refused to sign an EU association agreement and have called for early elections.
But Kiev's authorities say they will ban rallies in the heart of the city.
Ukrainian TV reported that a bid to prevent mass gatherings in Independence Square until next month was granted by a court on Saturday night.
Riot police had stormed the square in the early hours of Saturday when several hundred people were there. At least 31 people were taken into custody and a number of people were treated for injuries.
Kiev police chief Valery Koryak admitted afterwards that the "police did not come out of it looking good," but argued that missiles had been thrown at officers by some demonstrators.
In a statement read out on Ukrainian TV on Saturday night, President Yanukovych demanded an immediate investigation into the violence.
"I am deeply outraged by events that took place on Independence Square overnight. I condemn the actions which led to a confrontation and people's suffering."
He called for those responsible to be brought to account, but did not explicitly blame police. He also insisted that Ukrainians were "united by our choice of our common European future".
Several Western countries condemned the police intervention. US state department spokeswoman Jen Psaki condemned "the violence by government authorities against peaceful demonstrators".
Jailed opposition leader and ex-Prime Minister Yulia Tymoshenko urged Ukrainians "not to leave the authorities' actions unanswered" and preparations were made for a big rally on Sunday at 12:00 (10:00 GMT).
In a message read by her daughter on Saturday, Tymoshenko urged Ukrainians: "Fly, drive, walk to Kiev from all parts of Ukraine, but gather everyone on 1 December."
By Saturday evening demonstrators had regrouped in another square in the city centre, outside St Michael's Golden-Domed Monastery.
Another opposition party leader, Vitaly Klitschko, told people in the square that thousands of supporters were travelling from the western city of Lviv and other Ukrainian cities to take part in Sunday's rally.
Opposition parties say a "national resistance" HQ is to be set up, followed by a nationwide strike.
Members of Ukraine's political opposition had met for emergency talks after the dispersal from Independence Square.
"We have made a joint decision to form a national resistance taskforce and have begun preparing for an all-Ukrainian national strike," former Economy Minister Arseniy Yatsenyuk told reporters.
"Our demands are the resignation of [Interior Minister Vitali] Zakharchenko, an investigation of his actions and his trial, the resignation of the government and the president and early presidential and parliamentary polls.
"We... are calling on all civic activists, civil society and all those who care about Ukraine's future to fight the Viktor Yanukovych regime together."
President Yanukovych on 21 November suspended preparations for signing an EU association agreement that would have opened borders to goods and set the stage for an easing of travel restrictions.
At the end of a summit in Lithuania at which he had been due to sign the deal, EU leaders warned on Friday they would not tolerate Russian interference in the bloc's relations with former Soviet republics.
The summit did, however, reach provisional accords with Georgia and Moldova.
Mr Yanukovych said pressure from Moscow had led him to make his decision, arguing that Ukraine could not afford to sacrifice trade with Russia, which opposed the deal.
German Chancellor Angela Merkel said the door would always remain open for Ukraine.
Independence Square was the scene of the Orange Revolution in 2004, which saw Mr Yanukovych ousted and a Western-leaning government brought to power.
Mr Yanukovych was elected president five years later, narrowly defeating then-Prime Minister Tymoshenko, a leading figure of the Orange Revolution.
In 2011 she was sentenced to seven years in jail for abuse of office - a case widely criticised in the West as political revenge.
Tymoshenko has been on hunger strike since Monday over the failure to sign the EU agreement.