Stone-throwing protesters have clashed with police in central Kiev amid tensions over proposed changes to the constitution.
Thousands of protesters attacked the ruling-party headquarters and tried to march on parliament, reports said.
Tumultuous scenes have also been reported inside parliament, where opposition MPs are pushing for a vote on replacing the constitution.
The move is aimed at curbing President Viktor Yanukovych's powers.
The opposition had repeatedly warned that failure to act would further inflame mass anti-government protests - and these have reignited after a period of relative calm.
Ukraine's unrest began in November, when Mr Yanukovych backed away from an association and free trade deal with the European Union, and instead agreed to a large loan from Russia.
Moscow wants Ukraine to join the Russia-led customs union, which also has Belarus and Kazakhstan as members.
Both the EU and Russia have accused each other of interfering in Ukraine's affairs.
'Doors locked'
Tens of thousands of protesters trying to march on the parliament building have been blocked by lines of police vehicles, according to reports from Kiev.
Some are ripping up cobblestones to throw at police, and others are throwing smoke bombs, while police are responding with stun and smoke grenades, and rubber bullets.
Protesters also attacked the headquarters of President Yanukovych's party, t.