Calcutta: A 35-metre span of Majerhat bridge slumped around seven metres on Tuesday afternoon - with a minibus, five cars and three two-wheelers - to land on a canal and slushy ground beneath, in the second bridge collapse in the city in just over two years.

Soumen Bag, a resident of Behala's Silpara in his twenties, who was riding pillion on his friend's scooter, was killed. His friend Papai Roy, admitted in the intensive care unit at CMRI, is among 21 people injured.

Police and eyewitnesses said some labourers, working on the Joka-BBD Bag Metro project, might be trapped under the debris. The workers, who were on night shift, were believed to be resting in a tin shed under the bridge when it collapsed at 4.40pm. Close to midnight, the search operation was still on.

A New Alipore resident who takes the bridge regularly said he was "scared" while crossing it two days ago and had wondered if it could cave in. "There were five giant trailer trucks in front of me and the bridge was shaking... almost trembling. I thought for a moment if it could collapse," he said.

An official overseeing rescue operations said the 12-metre-wide bridge slumped gradually, a part of it landing on a canal and the rest on a vacant slushy ground, which reduced the impact and might have saved lives.

When the Vivekananda Road flyover collapsed in March 2016, 26 people had died.

Since the Majerhat bridge span fell flat, the cars and the bus did not topple and the passengers were safe. "Many came out on their own," the official said.

Uber driver Kamlesh Prasad Yadav, 40, was among them. "There was a loud thud. The road in front started tilting downwards like we see in a movie. I slammed the brakes in a reflex action but it was of no use," said Yadav, who was ferrying a woman and a child to Behala Parnasree. All three escaped unhurt. 

Anindya Mitra, who was working in the second-floor office of Keventer Agro, a stone's throw away, said the lights went out and it seemed the entire building shook. "We thought it was an earthquake but when we went out, people shouted that the bridge had collapsed," he said.

The crash has snapped the shortest route to the city centre for residents of New Alipore, Taratala, Behala, Thakurpukur and Joka, who will now have to brace for long detours to get to work and school.

The span that collapsed was immediately to the north of the one above the railway tracks. "The span didn't collapse on the tracks," a fire brigade official said, counting small mercies. Rail services on the Sealdah-Budge Budge line were hit.

Chief minister Mamata Banerjee, who is in Darjeeling, has set up a committee under chief secretary Moloy De to probe the accident and announced a compensation of Rs 5 lakh for the next of kin of the dead. The government will give Rs 50,000 each to the injured.

Tuesday's was the third bridge collapse under Mamata's regime. A chunk of the flyover connecting the Bypass and VIP Road had crashed in 2013.

Governor Keshari Nath Tripathi, mayor Sovan Chatterjee and urban development minister Firhad Hakim visited the accident site. The minister said the bridge was about 40 years old.

Disaster management teams of the police, fire brigade, civil defence and the National Disaster Response Force were part of the rescue operation.