The protesters were stopped during an Independence Day rally, UkranianJournal.com reported.
"This is what the government is for: to keep order," Hanna Herman, a top adviser to President Viktor Yanukovych, said. "There will be as many police forces as we need to make sure there is order in Kiev, to make sure there is no bloodshed."
Opposition groups conducted a rally in front of the Taras Shevchenko monument, then tried to march through Kiev toward the Yanukovych administration's offices, the Web site said.
"We will have a peaceful march toward the presidential administration to hand them a resolution of the rally," Oleksandr Turchynov, No. 2 in command in the Tymoshenko party, said before the activities. "Let them pack their bags."
The march was stopped by riot police several blocks from the monument. Protesters tried to break through police lines but failed, witnesses said.
Opposition groups had been trying to rally support for sustained protests since Tymoshenko was arrested Aug. 5 on abuse of office charges stemming from a natural gas agreement with Russia in January 2009.
Serhiy Sobolev, a member of Tymoshenko's Batkivshyna Party in Parliament, recently said there would be "very serious" protest actions in August and September that may cause a government shakeup and lead to early parliamentary elections in the fall.