Ukraine's political crisis has worsened after President Viktor Yushchenko fired the country's prosecutor general.
The two men have been jockeying for political power since 2004
But Svyatoslav Piskun vowed to defy the order, and his supporters occupied his offices in protest against the sacking.
The president's main rival, Prime Minister Viktor Yanukovych, has cut short his visit to southern Ukraine to hold an emergency cabinet meeting.
At the same time, Mr Yushchenko has summoned the heads of law-enforcement bodies for urgent talks.
The president and prime minister have been locked in a bitter power struggle for weeks after Mr Yushchenko dissolved the parliament in April and called a snap poll.
Mr Yanukovych and his governing coalition had initially rejected the move but later agreed in principle with the president to hold early elections. However, there has been no word on a date.
Mr Yushchenko originally said the vote would be held in May, but later moved the date back to 24 June after thousands of protesters from both sides took to the streets.
In his decree, Mr Yushchenko said Mr Piskun was dismissed for refusing to give up his seat in parliament as required by law.
But the prosecutor general said he was sacked because he had resisted a presidential order to take action against three Constitutional Court judges.
The judges had refused to step down after being dismissed by President Yushchenko earlier in April.
Reports of Mr Piskun's sacking prompted dozens of his supporters to march to his offices in the capital Kiev.
And in defiance of the presidential order, Ukraine's Interior Minister Vasyl Tsushko sent dozens of riot police to surround a building.
Mr Tsushko described the sacking as "an attempted coup".
Mr Yanukovych and his supporters had opposed Mr Yushchenko's call for elections since he dissolved parliament on 2 April.
The president had accused the prime minister of trying to usurp his power by illegally luring pro-Western lawmakers over to his coalition to increase his parliamentary majority.
The pair have been bitter rivals since Mr Yushchenko successfully overturned a disputed presidential election result in 2004 after mobilising thousands of supporters in central Kiev to protest against Mr Yanukovych's claim of victory.