ZAGREB, YUGOSLAVIA -- -- The president of the rebel Yugoslav republic of Croatia has said that three agents of the federal army, carrying concealed pistols and dressed in civilian clothes, were arrested this month while surreptitiously following his minister of interior.

President Franjo Tudjman said the three army men were captured by Croatian police after their car was spotted tailing Interior Minister Josip Boljkovac, who is in charge of a rapid buildup in the size and firepower of the Croatian police.

Tudjman's disclosure comes at a time of unprecedented tension between the federal army and the rebel northwest republics of Croatia and Slovenia, which are demanding a radical weakening of the Yugoslav federation.

Army leaders have said they will not tolerate the breakup of the country. The Yugoslav army is dominated by officers from Serbia, the largest republic and a bitter ethnic rival of Croatia.

In an interview last week in his office here, the Croatian president said many officers in the federal army are "opponents of the elected government in Croatia."

As he produced color snapshots of the three men arrested Sunday, Jan. 6, Tudjman charged that army agents have had ministers of his government under surveillance "for some time."

The arrested men were carrying a video camera, as well as a two-way radio, and produced their military identification papers only after they were taken to a police station, according to Croatian officials.

The three were turned over to federal military police. Croatian police identified them as Capt. Seid Smaij, Sgt. Zlatko Sestok and Zarko Vukosavjevic, whose military papers identified him as a member of an army intelligence unit. Repeated phone calls to army headquarters in Belgrade produced no comment on the Croatian president's statement.

Tudjman acknowledged that he is beefing up the Croatian police so they can defend Croatia from the army if it attempts to intervene in this republic, the second largest of the six that make up the federation.

Last week, the federal government in Belgrade empowered the army to disarm all illegal armed groups in the country within 10 days. The vaguely worded order does not make clear if the expanded police reserve in Croatia is considered illegal. Croatia rejected the order, saying it would resist an army intervention with "all available means."

In early December, the federal minister of defense threatened to use force to take weapons away from police and local militia in Croatia and Slovenia.

Gen. Veljko Kadijevic, in a statement that insisted that "socialism . . . belongs to the future" of Yugoslavia, said "those who created these armed forces will be held responsible before the law. There will be no bargaining and no compromises."

During a round of democratic elections in Yugoslavia last year, anti-Communist nationalist parties won sweeping victories in both Croatia and Slovenia. Both of these republics have pledged shifts to free-market economies.

In Serbia, however, former Communists won on a platform that blended nationalism with promises that the socialist state would not be dismantled.

Relations between the northwestern republics and Serbia were dealt a severe blow this month by the revelation of a huge banking scandal that threatens to undermine the country's monetary system.

The government of Serbian President Slobodan Milosevic illegally "borrowed" about $1.3 billion in federal bank reserves to pay pensions and salaries inside Serbia. Western diplomats say Milosevic used the money to help buy a big victory in elections last month.

Croatia's minister of defense revealed new details last week about how, if the federal army tries to intervene here, the police are preparing to defend the republic.

Martin Spegelj said about half of the 34,000 members of the Croatian police, the police reserve and "special units" have been equipped with new pistols and automatic weapons. Spegelj, who, like Tudjman, is a retired army general, said the weapons were bought abroad.

Spegelj said Croatia has established a policy under which nearly all police recruits must be ethnic Croats. Before Tudjman's election last spring, more than half of the police here had been ethnic Serbs. Serbs make up about 11 percent of the population of Croatia.

"We are remedying this disproportion. But we are not in a hurry," said Spegelj, adding that the government plans to use attrition -- not mass firing of Serbs -- to make sure that more than half of the police are ethnic Croats.

"Our police is an impressive force," Spegelj said. "But we must not forget self-defense by the people of Croatia. We have not abandoned this conception. This practically means that all citizens of Croatia would put themselves at the disposal of the Croatian government."

Spegelj said the republic is not providing arms to civilians but suggested that they are getting them on their own initiative. There are widespread reports of citizens buying rifles and pistols.

"The creativity and self-initiative of the people under such circumstances is incredible," said Spegelj.

A senior Western analyst described the Yugoslav army as probably "the only real army in Eastern Europe," saying it fought effectively against the Germans and Italians during World War II. It is well trained, well paid and well equipped with modern artillery, he added.

With more than 200,000 men under arms, the analyst said, the army could easily overwhelm the police and militia in Croatia and Slovenia.

There are major questions, however, about the willingness of the Yugoslav soldiers, the majority of whom are not Serbs, to obey a Serbian-dominated officer corps if action were taken against the rebel republics.

The CIA late last year predicted that Yugoslavia will break up by the end of 1991 and said there is a possibility of civil war.