They are demanding new elections, and the impeachment of the president.
Rallies continued through the night, with thousands gathered on the main square in the capital Kiev.
Police violently dispersed an opposition camp on Saturday, injuring dozens of people.
President Viktor Yanukovich said he was “outraged” by the police action.
The new rally is planned for Sunday at 12:00.
Jailed opposition leader and ex-PM Yulia Tymoshenko urged Ukrainians “not to leave the authorities’ actions unanswered”.
In a message read by her daughter on Saturday, Yulia Tymoshenko urged Ukrainians: “Fly, drive, walk to Kiev from all parts of Ukraine, but gather everyone on 1 December.”
“We can and should remove these authorities,” an opposition party leader Vitali Klitschko told thousands of demonstrators outside Kiev’s St Michael’s Golden-Domed Monastery.
“We should come out and show that we will not allow them to humiliate us, we will stand up for our rights.”
Vitali Klitschko said that supporters were travelling from the western city of Lviv and other Ukrainian cities to take part in Sunday’s rally.
Meanwhile, a court in Kiev has banned protested in Independence Square, European Square and at other locations in the city centre until January 7, 2014.
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