The bus, plying from Howrah to Gadiara, collided head-on with the trailer which suddenly veered to the wrong side of National Highway 6 near Mahisrekha bridge in the Uluberia area around 6.15 pm, police said.
Twenty-one passengers were declared brought dead at a hospital while around 30 of them rescued from the bus, which was reduced to a mangled frame of steel and wood, were rushed to different hospitals.
While several of them were referred to S S K M hospital in critical condition, some others were admitted to Howrah district hospital and Uluberia hospital.
Unconfirmed reports put the death toll at 30. Police and fire brigade personnel, along with locals, used gas cutters to bring out the bodies and the injured but the fate of the two drivers was not yet clear. Vehicular traffic on NH 6 crawled along one lane as rescue work went on in the adjacent cordoned portion of the road.
Union Railway Minister Mamata Banerjee, ministers of state Mukul Roy and Sultan Ahmed, the Trinamool Congress Member of Parliament from Uluberia, visited Uluberia hospital and inquired about the injured.
Roy later blamed the Communist Party of India-Marxist government for the poor infrastructure at the hospital.
Many of the passengers, travelling from Howrah to Gadiara village in the crowded bus, hailed from different localities under Shyampur and Bagnan police station areas.
Grieving relatives were seen crowding the corridors of Uluberia Hospital as the bodies, some in mutilated condition, were lying side by side. Utter chaos prevailed in the emergency ward as patients continued to pour in as the attending medical staff struggled to cope with the situation.
Many patients had to be referred to S S K M and Howrah Hospital. Several local organisations also deployed their ambulances to ferry the injured to hospitals in the district and Kolkata as the number of government-owned vehicles were inadequate.