Thousands of pro-Europe demonstrators are marching through the Ukrainian capital in protest at President Viktor Yanukovych's refusal to sign an EU trade deal.
Crowds gathered in a park before moving toward Kiev's Independence Square, in defiance of a court order banning rallies in the city centre.
They are demanding new elections, and the impeachment of the president.
Police violently dispersed activists on Saturday, injuring dozens of people.
The president said he was "outraged" by the violence, but Ukraine's interior minister warned on Sunday that police would respond if there were mass disturbances.
The BBC's David Stern, in Kiev, says the latest action may be bigger than last weekend's demonstration, which attracted 50,000-100,000 supporters.
Jailed opposition leader and ex-Prime Minister Yulia Tymoshenko urged Ukrainians "not to leave the authorities' actions unanswered".
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."
"We can and should remove these authorities," an opposition party leader Vitali Klitschko told thousands of demonstrators outside Kiev's St Michael's cathedral.
"We should come out and show that we will not allow them to humiliate us, we will stand up for our rights."
He said that supporters were travelling from the western city of Lviv and other Ukrainian cities to take part in Sunday's rally.
Hundreds of protesters occupied the square outside St Michael's cathedral overnight.
Meanwhile, a court in Kiev has banned protested in Independence Square, European Square and at other locations in the city centre until 7 January 2014.
The protests started more than a week ago after President Yanukovych 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.
Mr Yanukovych said pressure from Moscow had led to his decision, arguing that Ukraine could not afford to sacrifice trade with Russia, which opposed the deal.
Early on Saturday riot police stormed Independence Square beating protesters with truncheons. At least 31 people were taken into custody and a number of people were treated for injuries.
The president called for those responsible to be brought to account, but did not explicitly blame police.
Members of Ukraine's political opposition held emergency talks following the violence, and said a "national resistance" HQ would be set up, followed by a nationwide strike.
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.