Ukrainian President Viktor Yushchenko has been holding prolonged talks with his bitter rival PM Viktor Yanukovych, aimed at ending a political standoff.
Supporters from both sides are gathering in the capital, Kiev
The two men have so far failed to agree a date for an early election which they hope will resolve the crisis.
Mr Yushchenko earlier ordered several thousand interior ministry troops to head to Kiev.
But some 3,500 of the troops were prevented from reaching the capital by forces loyal to Mr Yanukovych.
The deputy head of the interior ministry - which is loyal to Mr Yanukovych - said that the troops were being led by a commander loyal to Mr Yushchenko and acting in defiance of ministry orders.
WHO CONTROLS WHAT?
President Yushchenko commands the 300,000-strong armed forces
Interior Ministry commands 40,000 troops and more than 200,000 police
Commander of Interior Ministry troops Oleksandr Kikhtenko is loyal to Mr Yushchenko
Mr Yushchenko said he was taking command of Interior Ministry troops on Friday
The troops are reported to have driven towards Kiev in a fleet of buses, and are thought to be carrying only riot gear and not lethal weapons.
"Moving the interior troops into the city is necessary to guarantee a calm life for the city, to prevent provocations," Ivan Plyushch, the head of the national security council, was quoted as saying on the presidential website.
But "practically all of them have been stopped in different places", AFP news agency quoted Mykhaylo Korniyenko as saying.
Mr Yushchenko said he was assuming control of the 40,000 Interior Ministry troops on Friday.
His order came a day after riot police - acting on the orders of the interior minister - defended the offices of Ukraine's prosecutor-general, a Yanukovych ally who Mr Yushchenko had sacked.
Saturday's talks finally started after several hours' delay.
Key lawmakers, including opposition leader Yulia Tymoshenko, also attended.
Both men agree that there has to be a new vote to resolve their dispute, our correspondent says.
Mr Yushchenko became president in January 2005 following the pro-democracy Orange Revolution, which overturned a rigged victory for Mr Yanukovych.
But Mr Yushchenko was forced to accept his rival as prime minister after his allies failed to win a majority in the March 2006 parliamentary election, and the two men have repeatedly clashed.
In April, Mr Yushchenko dissolved parliament and called a snap election, accusing his rival of trying to usurp his power.
Mr Yanukovych and his governing coalition initially rejected the move but later agreed in principle with the president to hold early elections. Now the two sides have to fix a date for the polls.
The president favours closer ties with the West, while the prime minister is seen as more pro-Russian.
There is growing international concern over the situation in the country. The EU has urged both parties to settle the deepening crisis though negotiation and not resort to violence.
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