Playing in front of 60,000-odd spectators, Pintu Mahata (20th) gave Mohun Bagan the lead, while Ugandan Henry Kisekka doubled the advantage around 10 minutes later.
East Bengal's Costa Rican World Cupper Johnny Acosta (45+2) pulled one back just before half-time with Laldanmawia Ralte (62nd) equalising in the second half which belonged to the red and gold brigade.
The result saw both teams go to 20 points each with Mohun Bagan ahead of East Bengal in the points table on goals scored.
Many people were stranded outside the stadium as the stands were full.
Mohun Bagan were off to a flying start from the word go with Kisekka warming rival goalkeeper Rakshit Dagar's gloves with a curling free kick.
Playing his first Kolkata derby, Acosta had a chance to put the red and golds in front but his header from a long freekick was blocked well by Shilton Paul under Mohun Bagan goal.
The green and maroon brigade were the more dominant side in the opening exchanges, making regular forays inside rival territory.
From one such raid, Mahata fired Mohun Bagan into the lead, turning in a cross from the far post as East Bengal midfielders showed no urgency to track back leaving free spaces inside the box.
Two minutes later, Kisekka almost doubled the advantage with his effort sailing just wide of the target.
Kisekka proved to be a constant menace inside East Bengal's box as Acosta, who played all three group matches in the World Cup in Russia, struggled to keep the Ugandan at bay.
Kisekka got his reward at the half hour mark when his ferocious drive beat Dagar all ends up after East Bengal's Mehtab Singh failed to intercept Arijit Bagui's ball from the right flank. The experienced midfielder was guilty of a poor touch deep inside his own half with Al Amna lurking and had it not been for a timely save by Shilton, it could have been 2-2. IANS