ZAGREB, Yugoslavia (AP) _ The leader of an ultranationalist Croatian opposition party claimed Monday that his deputy was assassinated, and threatened the government with retaliation.

Ante Paradzik, vice president of the Croatian Party of Rights, died in a hail of bullets Saturday evening at a police roadblock on the outskirts of Zagreb as he returned from a political rally.

Croatia's government has said the shooting resulted from an unfortunate error. It said Paradzik's car refused to stop at the checkpoint, and police believed the car contained Serb rebels.

But one of Paradzik's companions said that police officers manning the checkpoint first identified Paradzik, calmly stepped back a few feet and riddled the vehicle with their submachine guns.

Paradzik was hit 13 times.

''This was clearly a cold-blooded, pre-mediated political assassination,'' Dobroslav Paraga, president of the opposition far-right party, told a reporter.

He suggested that the government of President Franjo Tudjman was directly involved.

''Those responsible for this deed, whether Tudjman or one of his ministers, will have to pay,'' Paraga declared. He raised the specter of inter-Croat violence, saying many of his followers were seeking revenge.

Assistant Interior Minister Milan Brezak repeated the government stance Monday.

''The police mistakenly believeved they were terrorists,'' he told a reporter.

Paraga, 30, is a former dissident who spent four years in jail for his ultranationalist views in the early 1980s, when Communists still ruled Yugoslavia.

The increasingly popular party has repeatedly criticized Croatian authorities for being too soft in dealing with the Serb insurgents who have conquered a third of Croatia's territory in the past three months.

The party opposes any truce with the rebels and the federal army, while either remains in possession of any part of Croatia.

The party, which claims a membership of over 100,000, has organized and armed a militia - the Croatian Defense Force. It claims 10,000 armed volunteers.

Federal army officers in the blockaded Marshall Tito Barracks in Zagreb said in recent interviews they believed that members of the party militia, operating independently of the Croatian command, were attempting to provoke major confrontations by firing at the base.

Residents of buildings around the barracks said they saw sharpshooters wearing the militia's trademark red berets taking potshots at the barracks from neighboring high-rises.