Ukraine's power struggle has escalated and the country may be moving towards a snap election.
Street demonstrations in Kiev echo the 2004 "Orange Revolution"
The President, Viktor Yushchenko, is holding official talks with parliamentary leaders.
Mr Yushchenko has said he is prepared to dissolve parliament if no agreement is reached.
Supporters of his arch-rival, Prime Minister Viktor Yanukovych, have set up more than 200 tents in the park surrounding the parliament building.
They want the president to back down from his threat to call a snap election. Their protest camp sprang up after mass demonstrations.
In scenes reminiscent of the Orange Revolution two years ago, tens of thousands of Ukrainians took to the streets on Saturday.
Supporters from both sides organised rival rallies. The largest was held by people who back the pro-Western president.
PM Yanukovych (left) is locked in a power struggle with Mr Yushchenko
They are demanding that parliament is dissolved because they claim that the pro-Russian ruling coalition is usurping the president's power.
In essence, the Ukrainian political crisis is about where power should reside, says BBC analyst Steven Eke.
The president accuses the prime minister of expanding his majority in parliament in a way that no longer reflects what Ukrainians voted for in elections held a year ago.
The president's authority was undermined by two sets of constitutional changes that came into force after the Orange Revolution at the end of 2004.
The reforms were designed to ensure that future Ukrainian leaders would not fall into authoritarianism, as the previous president, Leonid Kuchma, had.
There is growing tension in Ukraine and an early election could well plunge the country into political chaos, the BBC's Helen Fawkes in Kiev says.