Renewed tensions between the president and Prime Minister Viktor Yanukovych have been building for weeks, and the president's move to take control over the troops, reflecting doubt on the loyalty of servicemen under the ministry's command, appeared to suggest rising concern over possible clashes.
A statement on the presidential Web site said Yushchenko's order for the troops to come under his command was necessary "to prevent using Interior Ministry troops in the interest of some political forces that cause a threat for Ukraine's national security."
But ministry spokesman Konstantin Stogniy said Yushchenko's order was illegal, and "fulfilling illegal orders is a crime."
The country's crisis intensified Thursday when Yushchenko fired Prosecutor General Svyatoslav Piskun, saying Piskun — as a member of parliament — couldn't serve simultaneously as chief prosecutor.
The Interior Ministry, which is led by a Yanukovych ally, responded by sending riot police to Piskun's office.
The Interior Ministry has about 32,000 troops and 220,000 regular policemen under its control; Yushchenko's order calls for his taking control only of the troops.
The Interior Ministry troops are led by Gen. Oleksandr Kihtenko, who is seen as a Yushchenko ally. The Interfax news agency cited Yushchenko aide Viktor Bondar as saying that the command had confirmed its readiness to follow Yushchenko's order, but the information could not immediately be confirmed.
The dismissal of Piskun, a member of Yanukovych's party, has aggravated already high tensions since Yushchenko's April 2 order dissolving parliament and calling early elections.
"It is again a violation of the Constitution and making such a decision is unacceptable. ... We need to immediately stop legal nihilism," Yanukovych said during an urgent meeting of his government.
Yushchenko said parliament's dissolution was necessary because Yanukovych and his coalition were trying to usurp presidential power. But parliament, where Yanukovych leads the majority coalition, has defied the order, calling it unconstitutional.
Yushchenko's dissolution order led to weeks of arguing and competing demonstrations between backers of the president and those of the premier.
The dispute complicated Thursday's dismissal of Piskun, since it was unclear whether parliament still legally exists. Piskun was initially defiant after Yushchenko announced the dismissal, but then said he would step aside once the order was officially published in the presidential register.
It was not immediately clear, however, whether Piskun had received or acknowledged the order.
Both Yushchenko and Yanukovych had agreed to respect the Constitutional Court's decision on the dissolution order. But the court has been deliberating on the matter for weeks, and the discussions were complicated by Yushchenko's orders to fire several of its judges, including the chief judge.
Yushchenko came to office in 2005 after the bitter Orange Revolution — massive protests that broke out after Yanukovych was counted as winner of a fraud-plagued presidential ballot. The Supreme Court annulled that vote and Yushchenko won the re-vote.
But Yushchenko's goal of instituting political and economic reforms in the ex-Soviet nation have run aground over factional fighting among his supporters. In last year's parliamentary elections, Yanukovych's party won the largest share of seats, apparently benefiting from wide voter dissatisfaction with the country's stalled reforms and internecine political sparring.
Yushchenko has repeatedly declared his aim of bringing Ukraine closer to the West, including eventual membership in NATO and the European Union. But the chronic political turmoil has hampered those aims and fed criticism of him for actions that are either ineffectual or unilateral.
Benita Ferrero-Waldner, the European Union's external affairs commissioner, meanwhile, voiced concern about the ongoing events in Ukraine.
"I call upon all political forces in Ukraine to do their utmost, on the basis of a constructive attitude and in a peaceful and lawful manner, to find a viable political compromise to solve the current political impasse and to refrain from any action which would further exacerbate the situation, especially by involving the security forces," says Ferrero-Waldner, in a statement from Brussels.