Stone-throwing protesters have clashed with police in central Kiev amid tensions over proposed changes to the constitution, BBC News reported.

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.

Russia's foreign ministry said the latest violence was a "direct result of connivance by Western politicians and European structures that have shut their eyes... on the aggressive actions of radical forces", reported Reuters news agency.

Moscow wants Ukraine to join the Russia-led customs union, which also has Belarus and Kazakhstan as members.

The EU and Russia have accused each other of interfering in Ukraine's affairs.