The riot police used tear gas when they dispersed the crowd of about 400 protesters who were demanding the resignation of President Viktor Yanukovych, demonstrators said.
The crowd was the remains of last night gathering that attracted some 10,000 people protesting Yanukovych's decision not to sign a long-anticipated association agreement with the European Union.
"It was horrible. We were holding a peaceful demonstration and they attacked us," said Lada Tromada. "They threw us away like garbage."
Ambulances were on the scene at Independence Square, and some demonstrators were seen bleeding from their heads and arms.
Police moved in on the demonstration at about 4:30 AM, said one of its organizers, Sergei Milnichenko.
Protests were held in Kiev over the past week since Yanukovych backed away from the agreement. It was to have been signed Friday at an EU summit in the capital of Lithuania, and the passing of that date sparked an especially large turnout of protesters.
Yanukovych abruptly changed course for integration with the EU last week when his government announced it was suspending preparations for signing the agreement.
The move angered many in Ukraine, where nearly half of the population of around 45 million favors closer ties with the EU.
Yanukovych argued that Ukraine can't afford to sacrifice trade with Russia, which has tried to block the deal by banning some of Ukraine's imports and threatening more trade sanctions.