The Ukrainian opposition hopes to muster tens of thousands of demonstrators on Sunday and give new momentum to demands for President Viktor Yanukovych to step down amid a row over ties with the European Union.
Three main opposition parties said they were establishing "a national resistance task force" after riot police brutally dispersed a rally of opposition supporters and wounded several dozen on Saturday.
The rally was broken up by baton-wielding police who attacked about 1,000 protesters on Independence Square in the capital Kiev in the early hours of Saturday morning.
About 10,000 people had gathered in central Kiev on Friday night calling for Yanukovych's dismissal after the president refused to sign a key political and free trade agreement with the EU.
The opposition called for a new protest in a central Kiev park after police surrounded Independence Square with metal barriers.
"We can and should remove these authorities," world heavyweight boxing champion Vitali Klitschko, the leader of the UDAR (Punch) party, told about 10,000 supporters on Saturday, announcing the new protest.
"We should come out and show that we will not allow them to humiliate us, we will stand up for our rights," he told the crowd nearly a week after mass protests broke out across Ukraine following the authorities' decision to scrap the EU deal that would have set the ex-Soviet nation on a path to European integration. The European Union has accused Kiev's old master Moscow of pressuring Ukraine, which is heavily dependent on Russian natural gas, to walk away from the deal.
On Saturday, current EU chair Lithuania said the use of force was "reprehensible" and EU foreign policy chief Catherine Ashton and Enlargement Commissioner Stefan Fuele called for a probe.
"Violence and intimidation should have no place in today's Ukraine," added US State Department spokeswoman Jen Psaki.
Amnesty International called on the Ukranian authorities to live up to their obligations to protect human rights.
Late Saturday, Yanukovych said in a statement he was "deeply outraged" by the use of force against the protesters and vowed that those responsible would be punished.
Opposition protesters massed Saturday outside an ancient, golden-domed church where several hundred demonstrators -- many of them out-of-towners -- had received sanctuary earlier in the day.
After the pre-dawn sweep, monks at the Mikhailovsky monastery gave the protesters, many with caked blood on their clothes and flags, first aid and food.
Everyday Ukrainians launched a drive to collect food and clothes for the protesters who were set to spend the night in the church.
Many said they felt betrayed both by the government and the opposition.
"Everyone abandoned us. We were left one on one with police batons," protester Igor Mitrov, his head bandaged and clothes blood-stained, told AFP.
"We do not know what to do next," said the protester, who arrived from the southern Crimean peninsula.
Yaroslava Fedorash, a 20-year-old protester from the western city of Lviv, said: "One young woman had her arm broken when the rally was dispersed. She's had it bandaged here."
One of the monks, speaking to AFP on condition of anonymity, said the demonstrators were allowed to remain in the church on the condition they refrained from smoking and drinking.
"Our monks will ensure they have food and hot tea."
Church leaders condemned the violence and called on both the authorities and the opposition to avoid further clashes.