Violence inside the Rajko Mitic stadium erupted late in the first half when rival supporters hurled seats and flares at each other, forcing riot police to move in.
Belgrade media reported that fans also clashed on a suburban motorway on the way to the stadium, grinding traffic to a halt before police arrived to restore order.
The fixture, which has a history of serious crowd trouble including fatalities, offered preciously few fireworks on the pitch until the last 10 minutes.
Striker Predrag Sikimic fired Red Star ahead with an unstoppable shot on the turn in the 83rd minute, hitting the top right corner from 13 metres.
Red Star defender Zoran Rendulic was sent off for a second bookable foul in the 90th minute and Partizan took full advantage with a fortuitous stoppage-time equaliser.
Their Brazilian midfielder Luiz Everton unleashed a swerving shot from 35 metres straight at the home side's keeper Damir Kahriman, who let it slip through his fingers when he should have made a comfortable save.
Everton was also shown the red card after taunting Red Star fans with exuberant celebrations and almost came to blows with the hosts' Portuguese striker Hugo Vieira, after exchanging insults while he was being escorted to the tunnel.
The result left Red Star top with 42 points from 31 games, 12 clear of closest rivals Cukaricki Belgrade and 14 ahead of third-placed Partizan with six games remaining.
The Serbian first division has adopted a new system with the top eight teams having their points tally halved at the end of the 30-round regular season.
They meet each other once on a round-robin basis in the championship playoffs.
(Editing by Toby Davis)
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