Police have fired rubber bullets at stone-throwing protesters in clashes near Ukraine's parliament, where a crucial session on constitutional reform is expected.
Protesters tried to get closer to the heavily fortified parliament building on Tuesday, leading police to retaliate with smoke bombs and rubber bullets.
About 20,000 protesters marched from Kiev's Independence Square to parliament to demand a vote on reforms to strip President Viktor Yanukovich of key powers, according to AFP news agency.
The protests were planned as Yanukovich looked likely to present his candidate for prime minister to parliament - a choice that will show whether he is ready to make more concessions to the opposition after 12 weeks of violent confrontation.
Tension remains high in Ukraine, despite the general prosecutor Viktor Pshonka's announcement on Sunday that criminal charges against activists would be dropped as part of an amnesty deal aimed at reducing friction between the government and its opponents.
The prosecutor's announcement came after protesters agreed to their part of the deal, which was to vacate government occupied buildings and allow traffic to access areas leading to state buildings in Kiev.
Unrest broke out last year when Yanukovich spurned a free trade agreement with the European Union and opted for a $15bn package of Russian credits and cheaper gas to prop up Ukraine's ailing economy in November.
The revolt escalated into national protests at perceived corruption in the Yanukovich administration, and has triggered a geopolitical tussle between East and West.
As Russia beckons with the aid package, the US and its allies have urged Yanukovich to return towards an IMF-backed deal with Europe.