Clashes have broken out in the Ukrainian capital as tens of thousands of demonstrators rallied despite new laws which aim to curb public protests.
Opposition politician Vitali Klitschko called for calm as pro-EU protesters, some with sticks and rocks, made for parliament despite police cordons.
The laws were passed with a quick show of hands on Thursday by MPs loyal to President Viktor Yanukovych.
The opposition accused the ruling party of a coup.
The president signed the bills into law soon afterwards.
US and EU officials have expressed deep concern at the new legislation.
Ukraine’s current anti-government movement began in protest at Mr Yanukovych’s decision in late November to pull out of a landmark treaty with the EU, but has expanded to demand his resignation.
Action plan demand
Sunday’s rally began peacefully after scuffles the previous day between pro-EU protesters and Yanukovych supporters in the city.
However, clashes erupted as demonstrators heading for parliament encountered cordons set up by police. Live TV pictures showed them attempting to overturn a bus commandeered by the police.
BBC Kiev correspondent David Stern said police fired stun grenades to clear the area and demonstrators responded with cries of «Shame!»
He described the situation as very combustible.
Earlier the rally on the main square heard a call from a former Ukrainian navy chief for members of the armed forces to defy «illegal» orders from those in power, Unian news agency reported.
Rear Adm Ihor Tenyukh, who was sacked by President Yanukovych in 2010, warned of the dangers posed by the «coup d’etat planned by the current authorities».
«Tomorrow the regime will enslave you too. Therefore we are calling on you to fulfil your military oath of loyalty to the Ukrainian people and not to the authorities who have gone off the rails,» he was quoted as saying.
Opposition leaders are under huge pressure to come up with an action plan, amid criticism from many activists that their campaign has been too passive.
The protesters have been camping out behind extensive barricades on the Euromaidan, as Independence Square has been dubbed, for nearly two months.
The mass demonstrations were initially triggered by President Yanukovych’s last-minute rejection of an EU deal under heavy pressure from Russia in November.
The protesters’ demands later widened to include the fight against what they said was widespread government corruption and abuse of power.
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