Belgrade — The new members of Yugoslavia's collective presidency have been in office only three weeks, but there are already signs of a tough new line. They came to power on May 15 in the midst of a crackdown against intellectuals and youth groups that has surprised many observers because of its extent and duration. Even some Western diplomats, known for their largely uncritical support of strategically important and nonaligned communist Yugoslavia, say they are ''watching the situation.''
Dissidents are quicker with their assessment of events. ''There's a new composition to the leadership, and a new policy,'' one said. ''We are the victims.''
The members of the nine-man presidency have changed for the first time since the passing of Josip Broz Tito four years ago. Only two of them have been there before; but all are experienced, old-guard politicians, like their predecessors.
The difference is that some of the new members are even more hard line. Among them are Stane Dolanc, former internal affairs minister in charge of security police, and Nikola Ljubicic, the former defense minister who was credited with crushing irredentist rioting among ethnic Albanians in Kosovo three years ago.
The only member of the presidency assessed as a liberal is Lazar Mojsov, the former foreign minister.
Yugoslavia's nine-man presidency, the only such collective leadership in the world, has a five-year mandate. It is headed by one of its members in an annual rotation. The members include a representative from each of Yugoslavia's six republics and two provinces, as well as the head of the League of Communists - as the communist party is known in Yugoslavia.
The new president of the presidency is Veselin Djuranovic, a Montenegrin. He is controversial because he served as prime minister from 1978 to 1982, the period when Yugoslavia acquired a large portion of its current $20 billion debt.
The crackdown on opposition voices began when the candidates for the collective presidency were announced early this spring. On April 20 a group of 28 people at a ''flying university'' session in Belgrade were arrested. The group was meeting to hear Yugoslavia's best-known dissident, Milovan Djilas, speak about the country's ethnic groups before World War II. All 28 were released within a short time and it was expected the incident would die down.
But subsequent events, including the apparent suicide of Radomir Radovic, the only worker in the ''group of 28,'' has brought world attention to the Yugoslavs. Human rights groups, including the US Helsinki Watch Committee, have made visits to Belgrade to investigate.
Intellectuals have circulated three petitions to the government asking for full disclosure of events surrounding Radovic's death. Four of the 28 were rejailed on suspicion of hostile propaganda or antistate activities. Two were arrested and confined for possessing petitions. Some of them have declared hunger strikes.
''That's the new element,'' one dissident said. ''The government never liked petitions before, but they were tolerated. This is a new warning.''
The Yugoslav press has responded to the crackdown with a deluge of articles defending the original arrests and calling the participants ''ghosts from the past'' who deserve ''the cold, dank air of underground cellar meetings.'' Mr. Dolanc, in a televised interview to mark the 40th anniversary of the security services, called the petitions ''a political fashion'' and complained that the petitioners circulated them to the Western press before giving them to the government.
At the same time this spring, the government cracked down on a number of youth publishing houses that are under investigation for carrying provocative articles. It also banned several issues of intellectual periodicals.
A Croatian summer school on the Adriatic was also banned. Among planned participants were former professors associated with the Marxist publication Praxis that was closed in 1975. Praxis was the only legal Yugoslav publication that regularly criticized the regime in the previous 11 years.
Also on May 15, Prime Minister Milka Planinc announced nine new Cabinet appointments in the largest such mid-term shuffle since World War II. She has two more years in office. Her appointees, chosen largely to preserve parity among the republics and provinces, bring less experience to the Cabinet than did previous members. Some observers say this represents ''a junior cabinet'' that is likely to be more subservient to the heavier weight presidency.
This spring, the presidency's power was substantially increased when the National Assembly passed a law that gives the presidency power to send federal police into republics and provinces, if it deems it necessary, instead of waiting for local officials to request such aid.
How strong the presidency will be and whether it will actually expand its role remain to be seen. But so far, at least in the handling of opposition, the indications are that it will use a strong hand.