Rival supporters of the two main opposing political factions are defying a court order and gathering for big rallies in the centre of the city.
The pro-Western President, Viktor Yushchenko, has threatened a snap election to end political deadlock.
The move is being resisted by the pro-Russian governing coalition.
There is a sea of blue, red and Ukrainian flags here in the capital, the BBC's Helen Fawkes reports from Kiev.
More than 10,0000 people are already attending the rally in support of Prime Minister Viktor Yanukovych, and are demanding the president back down from his threat to dissolve parliament.
Just a short distance away, are orange flags of presidential supporters whose rally is due to begin in a few hours' time.
They are demanding the early election, claiming that power is being usurped from the president illegally by the ruling coalition.
This political crisis is one of the most serious ever faced in Ukraine, our correspondent says.
Tensions are running high and there are fears there may be violent clashes.
Mr Yushchenko defeated Mr Yanukovych in the presidential election of January 2005, following the pro-democracy Orange Revolution.
However he was forced to accept him as the head of government after his allies failed to win a majority in the March 2006 parliamentary election.
On Saturday, he accused Mr Yanukovych of seeking to usurp power by trying to win pro-Western MPs over to his coalition and opposing his pro-Western policies.
"I want to say that unless the majority resumes its work on the basis of the constitution, I will sign a decree disbanding parliament," he told a meeting of his political party to loud applause.
He further accused the coalition of lacking "political will and desire to support stability".Güncelleme Tarihi: 20 Eylül 2018, 18:16