President Viktor Yushchenko, long at odds with his prime minister, issued a new threat to dissolve parliament yesterday as his supporters and detractors staged rallies more than two years after "Orange Revolution" upheavals.

The pro-Western Yushchenko has been sniping for months with Prime Minister Viktor Yanukovich, the man he defeated in 2004 after weeks of mass protests over election fraud.

The President, his powers cut by constitutional change, accused Yanukovich this week of illegally trying to expand the parliamentary coalition propping his government and suggested a new election would reveal what Ukrainians really wanted.

Yesterday, he was more explicit in saying he could dissolve the chamber and hold a new parliamentary poll.

He issued a series of demands to parliament, including a call to approve a law barring deputies from switching parties while in office. Our Ukraine members unanimously approved a call for a new parliamentary election. Thousands of backers of the more Moscow-friendly Yanukovich massed in a central square to denounce any suggestion of a new election in the ex-Soviet state.

A few hundred metres away, separated by police on horseback, supporters of Ukraine's Opposition, which has long called for a new election, were preparing a rally of their own.

Former Prime Minister Yulia Tymoshenko, the president's ally from the Orange Revolution and now the opposition's top figure, was to address demonstrators later.

Yushchenko reluctantly appointed Yanukovich prime minister after his own allies scored badly in a parliamentary election a year ago and were unable over four months to form a government.

Yanukovich agreed to leave intact the President's policies of seeking membership of NATO and the European Union. But his government has consistently chipped away at the President's authority, passing a law in December further curbing his powers.

Months of stalemate burst into new tensions with the defection to cabinet ranks last week of a top presidential ally.

Yushchenko asked the prime minister to allow the Constitutional Court to rule on a proposed ban on members of parliament crossing the floor to join rival parties.

Yanukovich can count on about 250 votes in the 450-seat assembly, and has vowed to expand his coalition to 300, a figure that could allow him to engineer further constitutional change.

He dismisses any notion that the President could dissolve the assembly, saying there are no legal grounds to do so.

The President has seen his personal support plummet to single figures in recent months.

An opinion poll published this week put Yanukovich's Regions Party, its power base in Russian-speaking eastern Ukraine, in the lead with more than 18 per cent support, with Tymoshenko's Opposition bloc second with more than 15 per cent. The President's Our Ukraine party lay third with seven per cent.