Leaders of Ukraine's leading opposition party, Batkivshchyna, said the demonstrators gathered in downtown Kiev as part of the Rise, Ukraine! campaign to call for the resignation of President Viktor Yanukovych.
The campaign is expected to end in mid-May with another massive rally in downtown Kiev, RIA Novosti reported Sunday.
"Some 20,000 people have gathered near a monument to Taras Shevchenko, where the rally takes place," Batkivshchyna leaders said in a statement.
Arseniy Yatsenyuk, Batkivshchyna's parliamentary faction leader; Udar Party leader Vitali Klitschko; and Oleg Tyagnibok, head of the All-Ukrainian Union Svoboda party were on hand at the rally.
Former Ukrainian Interior Minister Yuriy Lutsenko addressed the crowd, saying he plans to meet with Yulia Tymoshenko, jailed leader of Batkivshchyna.
Tymoshenko was once Yanukovych's main political opponent and was sentenced to seven years in prison for abuse of office for negotiating a gas deal between Russia and Ukraine, RIA Novosti reported.