The 40-year-old bridge in the crowded Alipore area caved in around 4.45 pm during rush hour over rail tracks. It runs over the Majerhat Railway Station and connects the city centre to Behala and other southern suburbs. 

While police confirmed the death of one person, fire brigade personnel engaged in the rescue work said 25 injured were pulled out from the rubble.

The injured have been admitted to SSKM and CMRI hospitals, the police said.

State Minister Chandrima Bhattacharya said she has heard about the death of five persons but it is yet to be confirmed.

NDRF DG Sanjay Kumar said earthmovers are being used.

"We have to take into account the safety aspect while clearing the debris", he said.

Another Minister Firhad Hakim said efforts are being made to find out any trapped person under the debris.

After the rescue work is over "we will undertake technical inquiry", he said.

An eyewitness said some people in a minibus and private cars have been trapped.

Chief Minister Mamata Banerjee said the first priority is to rescue the injured and those who are trapped.

She said a probe would be ordered to find out the cause of the collapse.

City mayor Sovan Chatterjee and senior police officers are at the accident site.

Circular Railway services and local train services in Budge Budge-Sealdah line were affected, an Eastern Railway spokesman said here.

The local train services were suspended from 4.45 pm, he said.

The bridge collapse hit traffic movement to and from vast areas in south-west Kolkata and adjacent South 24 Parganas district. 

Prime Minister Narendra Modi termed the incident as "deeply unfortunate" and hoped for the speedy recovery of the injured.

TMC leader Derek O'Brien tweeted, "Watching scenes of Majerhat bridge #Kolkata collapse. Devastating. Rescue teams, disaster management teams, Police CP, fire fighters already reach spot. Locals join. Rescue operations on. Prayers for those affected. Difficult to gauge enormity of tragedy as yet looking at visuals."