The incident threw normal life in large parts of South Kolkata and South 24 Parganas district off gear, as train services and vehicular traffic were disrupted.

West Bengal Chief Minister Mamata Banerjee, who confirmed one death, expressed apprehension about the fate of five-eight people who might be buried in the rubble. Nineteen injured people have been admitted in two city hospitals, she told the media in north Bengal's Darjeeling and announced a probe into the incident.

Locals, however, claimed five people have been killed in the bridge collapse.

At least 20-25 injured people have been rescued by fire officials while more people are feared trapped under the rubble, officials said.

"A part of the Majherhat bridge between Taratala and Mominpur collapsed around 4.30 p.m," a fire official said. Initially six fire tenders were pressed into service.

"At least 20-25 people have been rescued from the spot. It is possible that not many are trapped under the collapsed part of the bridge. But we need to remove the debris to find out what is underneath," another official working at the accident site said.

Four teams of the National Disaster Response Force (NDRF), fire brigade and personnel of disaster management teams are at the spot. Cranes have been deployed and gas cutters are being used for the rescue operation.

Three columns of army personnel are also present at the spot, but there has been no official deployment as no such request was received from the state government, Eastern Command sources told IANS.

The accident took place on a day the city received 72.1 mm of rainfall since morning. The rescue operation was impeded due to occasional rain in the evening and lack of daylight on a rainy day.

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.

"We have heard one person died so far. We are trying to get back but it takes close to four hours even to get down to the plains from the Darjeeling hills on a rainy day. Also no flights are available right now.

"We are constantly monitoring the situation. We are initiating a probe so that everything can be found out. Steps are being taken on an emergency footing to rescue those who are trapped," said Banerjee.

She added: "We have heard that four-five labourers used to stay in a temporary tin shed under the bridge. If they were there during the collapse, it is possible that something unfortunate may have happened to them.

"However, all the pedestrians and passengers in the public and private vehicles could be rescued. We are concerned about the lives of five to eight people who might have got trapped underneath."

Many people were still reported missing. Workers of the Metro Rail project which was going on beside the collapsed bridge, claimed that three labourers have so far not been traced.

The incident disrupted public transport routes and overall traffic movement through the otherwise bustling Diamond Harbour Road in Taratala was suspended.

Train movement through Majerhat was also suspended temporarily as a precautionary measure after a signal post on the track was damaged by the debris of the collapsed bridge.

Train services on Budge Budge-Sealdah line may be restored after ensuring the safety in train running, a railway official said.

Prime Minister Narendra Modi expressed grief.

"The collapse of a part of a bridge in Kolkata is deeply unfortunate. My thoughts are with the families of the victims. I pray that those who are injured recover at the earliest," Modi said in a tweet.

State Governor Keshari Nath Tripathi also visited the location and later asked the government to take corrective measures for proper maintenance of bridges.

Urban Development Minister Firhad Hakim, who rushed to the spot and supervised the rescue operation, said rescuers who went under the bridge did not find any trapped persons.

"They have not found any trapped persons. But they are still trying to ascertain whether anybody is trapped," he said. Forensic experts also went to the location.

According to a state official, the 40-year-old bridge was constructed by the Kolkata Port Trust and the bridge was maintained by the State Public Works Department.

Tuesday's incident revived the dreadful memory of the Posta flyover collapse here two years ago. Twenty six people were killed when the under-construction Vivekananda Flyover collapsed in the city's Posta area on March 31, 2016.