The victim Sajal Kanjilal - a resident of South Kolkata's Kasba area - was taken to state-run SSKM hospital where he was declared dead.
Eyewitnesses said the middle-aged victim tried to board the Kavi Subhas-bound train around 6.40 p.m. at Park Street station, but could not get in fully due to a heavy rush of passengers.
He had only managed to slip one of his hands inside the compartment, when the sliding doors closed and his hand got stuck. To the horror of onlookers, the train started moving towards Maidan station, dragging the man whose body was hanging outside.
After the train had moved some distance, the shouts of other train passengers alerted the driver, who applied the emergency brake, and the victim fell on the track below with his face down.
The train came to a stop with four coaches inside the tunnel and the remaining four on the platform area.
The authorities then disconnected the power supply to the train as a safety measure.
A passenger Sudip Roy said soon after the train started moving towards Maidan there was a burning smell and the train came to a halt.
Eyewitnesses said the victim lay in the tunnel for quite some time before security personnel ran and removed him.
A Kolkata Metro railway spokesperson described the incident as "unfortunate" and said a three-member inquiry committee has been formed to view the CCTV footage and the circumstances leading to the incident.
The train's motorman and the guard have been suspended.
The spokesperson said this is the first time such an incident has happened in the 35-year history of the Kolkata Metro.
Calling the incident "shocking", Chief Minister Mamata Banerjee alleged that the Kolkata Metro was now "most neglected".
"I had formed a separate zone for the Metro railway. Then we used to bring new rakes, and keep a tight vigil," she said.
Assailing the BJP government at the Centre, she said "no maintenance work is now carried out in the Metro railway".
She promised all help to the family of the deceased. "If need be, one member of the family will be given a job."
( With inputs from IANS )