A power struggle inside the government in Ukraine has brought tens of thousands of demonstrators out on to the streets of the capital, Kiev.
Amid a sea of flags, rival supporters - some supporting the president, others the PM - gathered in the city centre, defying a court order banning protests.
Pro-Western President Viktor Yushchenko has threatened to dissolve parliament and call new elections.
The pro-Russian governing coalition led by Viktor Yanukovych resists the move.
The two rallies each attracted about 30,000 people, local police say.
Those at the rally in support of Mr Yanukovych heard calls for the president to back down.
Just a short distance away, where the Orange Revolution took place in late 2004, there were calls for an early election and claims that power was being usurped from the president illegally by the ruling coalition.
The rival rallies went ahead even though a court had banned protests in the capital.
Police in bullet-proof vests patrolled the barriers set up to separate the rival protesters, and asked people moving from one square to another to put away political flags and scarves.
This political crisis is one of the most serious ever faced in Ukraine, with both sides determined to get their way even if it means more protests, says the BBC's Helen Fawkes in Kiev.
"It is not the best way to make revolution, twice, but today's situation... cannot be resolved without revolution; today's government is not the voice of the people, it only wants to make some profit," one Yushchenko supporter told the BBC.
Mr Yushchenko defeated Mr Yanukovych in the presidential election of January 2005, following the pro-democracy Orange Revolution.
Mr Yushchenko accuses his rival of seeking to usurp power
But 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.
Mr Yushchenko has expressed concern that Mr Yanukovych could strengthen his parliamentary majority, enabling him to override presidential vetoes and amend the constitution.
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 the "political will and desire to support stability".
I was out on Maidan this evening together with the 100,000 presidential supporters. The atmosphere today was very different to that of the Orange Revolution two years ago - the people on both sides are sick and tired of the continuous squabbling and the incompetent and corrupt parliamentarians. Yushchenko was the people's hope two years ago but Yulia Tymoshenko and Yanukovych have been slowly destroying him trying to take all the power unto themselves. President Yushchenko has suffered the most - firstly by being poisoned and now being constantly stabbed in the back by friends and foes alike.
Andriy Hunder, Kyiv, Ukraine
I am a Kiev native and want to express my support of Yanukovych, who clearly represents a majority of Ukrainian people. The results of elections speak for itself. Yushenko is in denial, worse, he is illegally using his power, which he doesn't deserve anyway, to create a chaos in the country. He must go, to the West where he belongs, to his masters, who pay him and his gang. Tymoshenko should join him.
pavel gatynya, kiev
When I was going to Kiev's Independence Square today, I was a bit anxious, thinking there would be not many people. Fortunately, I was wrong - there were thousands and thousands of opposition supporters. Yesterday, I doubted President Yushchenko would disband this awful parliament but today, after that spectacular rally, I am sure he will dissolve it.
andrew, Kyiv, Ukraine
I'm in Kiev on a business trip and just couldn't resist temptation to visit Maidan Nezhalezhnosti (Independence Square) and European Square. There are lot of people there and still more are gathering, everything is still peaceful. Rhetoric of opposition's speeches is somewhat funny. They vilify the Prime Minister as if he is the ultimate evil on Earth. In my opinion, Ukraine is divided: one part of Ukraine wants to be a part of EU and another part wants to be independent state with close ties to Russia. There are many rifts between these two parties. For example, Russian language is not a state language even though 75% of people speak it fluently and about 40% speak it exclusively. Another issue is Crimea - it was Russian territory for centuries until Khrushchev gave it to Ukraine as a present in sixties. Naturally, Russian people in Crimea want to speak their own language and have a greater degree of autonomy.
Alex Besogonov, Izhevsk, Russia
I'm English but live in Kiev. Its been a lovely sunny day here. great to sit in the park and drink a beer. Don't believe that Yanukovich is the baddie and Yushchenko and Tymoshenko the saviours. they are all busy making money out of this. meanwhile life goes on, and on a day like this it's pretty good!
Keith smith, Kiev, Ukraine
Me and my mates (broad range of professions - civil servants to tobacco company agents) were participating in the opposition's rally. Having spent the first week of 2004 protest at fresh air (meaning Maidan) and thereafter experiencing so much disappointment for those petty intrigues former heroes immediately fell in, everyone of us wasn't sure till the afternoon on whether to join the protests led by the people who have failed to keep close to their promises in that nasty way. But still we all made it (the decision) and the old company was back to Maidan. The reason is that Tymoshenko and Our Ukraine (pro-presidential one) is a chance, the slightest of hopes is still a hope. A chance for us, our children and the country to develop. Otherwise in a couple of years we won't be able to choose. Each one of us has got enough evidence to be sure about it.
Pro Yushchenko! Ukraine has suffered enough from greedy politics and long noses from the Russian parliament. We need a stable government that will carry us out of dark days farther into Europe. We don't need Russia and we don't need Yanukovych.
Paul Baidatsky, Tryckavetc', Lviv Region, Ukraine
Living near the Presidential Administration I have noticed that in the last few days there has been an increasing build-up in police presence, including buses of Berkut special forces, parked in some of the court yards lining Liuteranska Street. About 10 minutes ago according to the Ukrainska Pravda website, Vasyl Kyselovm, Deputy Head of the Party of Regions and a loose cannon in my opinion, called on Party of Regions supporters to gather around the Cabinet of Ministers Building and Verkhovna Rada on Monday and Tuesday in order to not allow the overthrowing of the State. I am not sure what will happen today, but I am sure that the police who are out today are of a different ilk than those who were on Majdan in 2004, and they are definitely taking orders from a whole different group of people.
Vasyl Pawlowsky, Kyiv
This time it is highly difficult for the West to buy so-called democracy again... I hope our people finally realise that Ukraine is needed by the West only as a cheap resources supplier, consumer of second-hand goods and different types of waste, and as a fore post base to fight Russia. I hope people's wisdom wins against Western $ this time!
Aleksandr Borisov, Kharkiv, Ukraine
When are we going to see a fresh face in Ukrainian politics? How long will we have to watch Yushchenko, Yanukovych & Tymoshenko struggle for power? They are all crooks.
Sergey, New York, formerly from Ukraine
It is time for Yushchenko to step down. He is incompatible with the dominant coalition. He is the cause of the government's instability. I do not understand his desire to embrace Western standards. At what benefit? Yushchenko can operate without dictation from the economic fascists of the West, which would serve Ukraine better. I hope a new revolution is started where the push is for a government that does not play to the desires of the Western media. Because we may not embrace Western ideology does not make our way wrong.
Andrej Wyzchecknyk, Saint Petersburg, originally of Kyiv, Ukraine
Pseudo politicians such as Moroz, Yanukovych and Akhmetov should face trial for betrayal of people of Ukraine. They are simply ripping the country off, selling national interests (WTO, Nato, EU) for their very own profit (preferences in Russia for own companies). For how long can thefts keep power of a country desperate for change? Ukraine is more than Donetsk City Council - it's a homeland for 50 million and roots for 80 million people outside of the country.
the Cossack, Toronto,Canada - Dnipropetrovsk, Ukraine
Yanukovych did nothing but triple the utility bills and attempt to usurp power by forming a parliamentary constitutional majority of 300 MPs in a non-constitutional way to be able to ignore the presidential vetoes and alter the constitution the way he wants. I think snap elections are the only ultimate way of solving the crisis - let the people decide whom they want to govern the country.
Oleh, Stryi, Ukraine
I clearly support Yanukovych as he has brought in growth in the economy and relative stability. Prosperity is once again coming to the region for the first time since the demise of the Soviet Union.