There have been protests in Kiev against President Viktor Yushchenko's attempt to seize control of the interior ministry's security force.
The president's rival, Prime Minister Viktor Yanukovych, said the move meant the "use of force scenario" had begun.
The row over control of the interior ministry troops came a day after riot police seized the offices of sacked prosecutor general, Svyatoslav Piskun.
The crisis has been deepening since April, when Mr Yushchenko dissolved parliament and called a snap election.
The president's supporters have accused the PM of trying to usurp his power by illegally luring pro-Western lawmakers over to his coalition to increase his parliamentary majority.
The National Security Council will hold an emergency meeting on Saturday to try to defuse the crisis, reports say.