Yugoslav authorities have reacted sharply to what they regard as a marked increase in dissent activities coinciding with marshal Tito's illness and death. Milovan Djilas, a former vice president, has been singled out for particularly harsh criticism.

Since president Tito died two months ago, a series of political trials have been held demonstrating the new leadership's determination to keep a tight rein on dissent. At the same time, 36 prominent Belgrade intellectuals have appealed for the release of all imprisoned political prisoners in Yugoslavia, at present believed to number around 500.

The latest attack on Djilas, once one of Tito's closest aides and friends, was contained in an article carried yesterday by the Belgrade daily Politika. It described his activities as illegal and accused him of maintaining contacts with all kinds of anti-Yugoslav emigres, including right-wing nationalists and pro-soviet communists.

Contacted by foreign journalists at his home in central Belgrade, Djilas, 69, said he feared the latest article could signal a new campaign for his arrest supported by hard-line elements in the new collective leadership.

The public criticism of Djilas, which was clearly officially inspired, reflects growing disquiet within the government about a series of interviews he has given to Western newspapers and magazines in the last two months. Yugoslav leaders are also understood to be angry at his decision to publish a personal assessment of Tito.The book is due to appear in the United States and West Germany later this month.

In its three-column article, entitled"The Blindness of a Rengade," Politika said Yugoslavia did not fear censure abroad for alleged violations of human rights. Referring to Djilas, it said: "He thinks he will be allowed to write and speak as he likes, against the norms of this society."

Djilas has already served three prison terms totaling nine years for hostile propaganda. He fell from power in 1954 when he advocated greater democracy in Yugoslavia and was last released from jail at the end of 1966.

Asked about Djilas' case recently in a background interview, a top Yugoslav Communist Party official said the leadership's room for maneuver was limited. "I fear our patience may soon be exhausted. We simply cannot use up all our energy defending ourselves from various emgire attacks. We have other, more important tasks," he said.

Noting that Djilas had advocated a multiparty system in Yugoslavia, the official said such a system would have fatal consequences, particularly in view of the country's delicate geopolitical balance between East and West.

According to official statistics, the number of convictions for political offenses has risen in the first six months of this year. This is attributed to an increase in antistate activity, both inside and outside the country, during Tito's long illness and after his death.

The most prominent cases have concerned a group that allegedly planned terrorist acts in the northern republic of Croatia and another that distributed pamphlets in the Albanian-inhabited province of Kosovo, urging its unification with the nation of Albania.

In a magazine interview, the chief federal prosecutor revealed that nine out of 10 political prisoners in Yugoslavia have been convicted of so-called "verbal crimes," and the vast majority of these have involved simple curses. It appears that many Yugoslav officials are asking why Djilas should not be subjected to the same criteria.