Police used tear gas and smoke bombs in a bid to disperse hundreds of people who sought to storm police cordons near the Verkhovna Rada parliament in the capital.
Many of the demonstrators wore pots and colanders on their heads while others sported ski, medical and carnival masks to mock new legislation which forbids protesters from covering their faces.
But in a sign of the protest movement's growing impatience, key opposition leaders were jeered at during the main rally on Independence Square for their perceived inability to mount a stronger challenge to President Viktor Yanukovych.
Some demonstrators rocked police buses outside the Verkhovna Rada and set one of them on fire as police used tear gas against them.
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The flames from the blazing police bus lit up the evening sky while the thud of smoke bombs and stun grenades echoed around.
At least two protesters were hurt, an AFP correspondent saw.
Top opposition leader and former boxing champion Vitali Klitschko urged the crowd not to provoke the police, but many pressed on further towards the parliament and nearby government buildings.
"We declare the legislation adopted on Thursday illegal," Klitschko told the sea of chanting and flag-waving anti-government protesters as they filled Independence Square and its surrounding streets to bursting point.
"Yanukovych and his stooges want to steal our country," he said.
Yanukovych, 63, on Friday signed into law tough legislation that bans virtually all forms of protests in a move the opposition called a power grab and the West said was anti-democratic.
The new laws allow the authorities to jail those who blockade public buildings for up to five years and permit the arrest of protesters who wear masks or helmets.