The result left Partizan in the driving seat with 33 points from 12 matches, eight more than bitter city rivals Red Star who slipped to a 1-0 defeat at neighbours OFK.
The second successive league defeat for 1991 European Cup winners Red Star piled the pressure on the club's under-fire president Vladan Lukic to step down ahead of next Saturday's potentially explosive home derby with Partizan.
Lukic first fell out with the fans after club great Robert Prosinecki, one of the 1991 generation's key players and a former Croatia midfielder, resigned in August as coach after coming to loggerheads with the president.
Last weekend's 2-1 home defeat by Jagodina was followed by another poor performance against OFK, whose striker Petar Grbic shimmied his marker with a stepover and steered a 59th-minute winner on a sunny afternoon in central Belgrade.
The home side could have won by a bigger margin but for slack finishing, while Red Star fans jeered their team off the pitch and called for Lukic to step down.
"We didn't look like a team, we couldn't do anything right today and we are heading into the derby against Partizan with a sour atmosphere in the dressing room," Red Star coach Aleksandar Jankovic told Arenasport television in a pitch-side interview.
"We have a week to get back on the rails and keep the title race alive," he said.
Bulgaria defender Ivan Ivanov headed Partizan into a 24th-minute lead after meeting Nemanja Tomic's free kick which came off the crossbar and teenage forward Lazar Markovic sealed the win in the second half after he capped a flowing team move with a crisp first time shot.
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