BELGRADE, Serbia — The pro-European party that has led Serbia since 2012 appeared headed for a big victory in a snap election on Sunday, even as right-wing nationalists made gains.

In the election, Serbia’s third in four years, Prime Minister Aleksandar Vucic’s Serbian Progressive Party had secured 49 percent of the vote with 82 percent of the ballots counted, according to preliminary results released by the Center for Free Elections and Democracy, an independent monitoring organization, the Serbian state broadcaster reported.

Mr. Vucic’s coalition partner, the Socialist Party of Serbia, was projected to win 11 percent of the vote, according to the initial results.

A pro-Russian party led by Vojislav Seselj, who was acquitted at The Hague last month of war crimes and crimes against humanity over his role in the Balkan wars of the 1990s, was receiving 8 percent of the vote, enough to secure representation in Parliament. Four other parties or coalitions of parties also appeared to win enough votes to gain seats in Parliament.