The Majerhat bridge, thought to be older than 40 years, is located on Diamond Harbour Road in the Alipore area, and connects central Kolkata with the densely populated Behala region of South Kolkata.
Another minister Firhad Hakim said there has been no casualty in the incident, adding that all those trapped under the debris have been rescued, and six injured have been admitted to the nearby hospital.
ARMY, Navy and CISF teams were deployed to assist the NDRF, Fire Brigade and the Kolkata Police in the rescue operation.
The area near the site had been dug up for a metro railway project and construction materials are strewn all over the place.
According to her, the state audits bridges every six months to know know whether renovation is required.
TMC leader Derek O'Brien tweeted, "Watching scenes of Majerhat bridge #Kolkata collapse".
Mamata Banerjee on Majerhat Bridge Collapse in Kolkata: "We are very anxious".
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The Majerhat bridge reportedly collapsed around 4:45pm.
"We dug two manholes into the debris and rescuers descended into them looking for survivors, but did not find anyone there".
Banerjee, now on a trip to Darjeeling, expressed concern over the incident and said her government will initiate a probe into the cause of the bridge collapse.
"If the labourers were in the hut at the time of the incident, some lives may have been lost", the Trinamool Congress chief told reporters.
"The collapse of a part of a bridge in Kolkata is deeply unfortunate". Cranes have been deployed and gas cutters are being used for the rescue operation. "Hard to gauge enormity of tragedy as yet looking at visuals".
After a portion of the Majerhat bridge collapsed on the tracks, train services along the Circular Railway and Sealdah-Budge Budge line were temporarily suspended as a precautionary measure, Eastern Railway spokesperson RN Mahapatra said.
Opposition parties blamed the Trinamool Congress government and said its "callousness towards the fix of old bridges" was the main reason for the collapse.