KISHINEV, U.S.S.R. (AP) _ Moldavian nationalist leader Mircea Snegur won a landslide victory as the only candidate in the republic's first presidential vote but faced more ethnic trouble in the mixed Romanian and Slavic republic.

Snegur, a former Communist who led Moldavia's independence drive, captured 98.17 percent of the votes, Central Election Committee chairman Vladimir Kiktenko said today.

Kiktenko said 82.9 percent of Moldavia's 2.4 million eligible voters turned out. But activists among the mostly Russian and Ukrainian population of the Trans-Dneister region of eastern Moldavia tried to boycott the election.

According to the Moldavia Press Information Agency, ethnic Russians blocked at least one polling station in the region, editor Fyodor Usika said.

The Tass news agency, quoting the Moldavian Interior Ministry, said 64 armed groups had been formed to prevent voters from casting ballots in the region. However, Usika said that except for the one blocked polling station, the election appeared to be going smoothly.

In the nearby republic of Byelorussia on Sunday, leaders of the Soviet Union's three Slavic republics issued a statement warning Moldavians to respect rights of minorities, Tass reported.

''We are convinced that all ... problems in this republic, including the question of observation of rights of national minorities, must be resolved through peaceful means,'' said the leaders of Russia, Ukraine and Byelorussia after creating a new commonwealth of independent states.

Snegur, after casting his ballot, also said the way to solve the ethnic conflict was ''through the display of reason and calmness.''

Two-thirds of Moldavians are ethnic Romanians and many want to unite with Romania, which controlled most of Moldavia until the Soviets annexed it in World War II.

Ethnic tension rose soon after a nationalist government took power in Kishinev two years ago and required everybody to speak Romanian. Trans- Dniester, with about a sixth of Moldavia's 4.3 million people, proclaimed itself a separate republic last year.

Trans-Dneister separatists argue that their region, which was incorporated into Moldavia by Stalin, should revert to Ukraine.

But Moldavia is loathe to abandon Trans-Dneister's relatively advanced industrial base. The area also supplies most of Moldavia's energy.

Moldavia also faces separatist sentiment in the southern Gagauzia region, populated by 135,000 ethnic Turks, and in Kishinev itself, where nearly half the population is not Romanian.

The 51-year-old Snegur was a Communist party functionary for nine years before becoming head of the republic's Supreme Soviet in 1989. Parliament elected him president a year later, and he then abandoned the Communist party.