The demonstration, which organizers hoped will attract 100,000 people, was called to pressure authorities to reconsider the increase in tariffs, which are slated to go up 85 percent for natural gas.
Yulia Tymoshenko, who is expected to be sworn in as the new prime minister this week, has vowed to reconsider the deal with Russia, but all politicians acknowledge that the heavily subsidized prices Ukraine used to enjoy are history, the AP reports.
Electricity prices have also risen 25 percent and train tickets by an average of 43 percent - part of an ongoing effort to reduce public sector debt in this ex-Soviet republic.
But residents - many struggling with low pensions and low state salaries - argue that they can't afford the increases. The protests were organized by Federation of Ukrainian Trade Unions and miners, nurses, and other state workers from across the country began flooding into Kiev early in the day.