Tens of thousands of protesters are expected on the streets of Kiev in Ukraine for rival rallies amid an escalating political crisis.
A power struggle inside the government in Ukraine has brought thousands of demonstrators out on to the streets in the capital, Kiev.
The pro-Western President, Viktor Yushchenko, has threatened to call a snap election.
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.
He is caught in a growing power struggle with Prime Minister Viktor Yanukovych, who is more pro-Russian.
The pro-Western President, Viktor Yushchenko, has threatened a snap election to end political deadlock.
Mr Yushchenko's supporters will take to the streets again, just as they did two years ago during the Orange Revolution.
The move is being resisted by the pro-Russian governing coalition.
Then, mass protests brought him to office. Now, President Yushchenko's supporters want him to call a new election and dissolve parliament, which is currently controlled by the arch-enemy of the Orange Revolution, Mr Yanukovych.
There is a sea of blue, red and Ukrainian flags here in the capital, the BBC's Helen Fawkes reports from Kiev.
The rally has been organised by the opposition. It claims the pro-Russian ruling coalition is illegally trying to seize power.
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.
Recently a number of MPs who support the president have defected, with more expected to follow suit.
Just a short distance away, are orange flags of presidential supporters whose rally is due to begin in a few hours' time.
But the ruling coalition wants the president to back down from his threat to call an early election, and it is also holding a protest in Kiev on Saturday.
They are demanding the early election, claiming that power is being usurped from the president illegally by the ruling coalition.
This will take place near the opposition rally and at around the same time.
This political crisis is one of the most serious ever faced in Ukraine, our correspondent says.
Tensions are running high, and with thousands of protesters expected, there is concern that there may be violent clashes.
Tensions are running high and there are fears there may be violent clashes.
Bitter rivals
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".
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