ZAGREB, Yugoslavia (AP) _ Rock-throwing protesters broke through a police cordon today and forced the postponement of the trial of Croatia's defense minister.

The minister, Martin Spegelj, and seven others are accused of plotting an armed insurrection against Yugoslavia. If convicted by the military court, they could receive life in prison.

The trial in the Croatian capital, Zagreb, is considered likely to further strain relations between the federal army, which has supported continued central control, and the independence-minded republic.

Meanwhile, Soviet Foreign Minister Alexandar Bessmertnykh arrived in the federal capital, Belgrade, today, beginning a two-day visit to Yugoslavia.

Bessmertnykh was to meet Yugoslav President Borisav Jovic and Premier Ante Markovic to discuss Soviet-Yugoslav relations, as well as growing ethnic strife in both countries, the Tanjug news agency reported.

In Zagreb, 2,000 protesters gathered in front of the court building before the defense minister's trial was to begin this morning.

The demonstrators, who chanted ''Freedom 3/8'' and waved Croatian flags, pushed aside riot police and stoned the building as the trial got under way. The stones broke several windows.

The presiding judge, Capt. Mile Vignjevic then declared the proceedings ''indefinitely adjourned.''

Spegelj, a retired general who last year was named defense minister in Croatia, is being tried in absentia. The seven others charged with him have been held in a military stockade since February.

Croatia has refused to surrender Spegelj to federal military judicial authorities, who have accused him of buying thousands of weapons to prepare for an uprising against the Communist-dominated army.

Last January the army broadcast a secretly filmed documentary purporting to show Spegelj conspiring to illegally import about 10,000 Kalashnikov rifles and to organize resistance to the army.

Spegelj denied the footage was authentic, and said Croatia legally purchased the guns for its regular police force.

A military prosecutor ordered Spegelj's arrest.

Croatia maintains he has immunity as a member of the republic's Cabinet. Spegelj has moved openly in public despite the military's call for his arrest, but has been under heavy police guard.

The federal army's officer corps is largely ethnic Serbian and pro- Communist. It is at odds with the pro-Western governments of Slovenia and Croatia over Yugoslavi'a future.

Croatia and Slovenia want to transform Yugoslavia into a political and economic coalition of independent nations, while Serbia and the military leadership want a centralized federation.