The crime evoked disgust across the state and opposition parties attacked the ruling Trinamool Congress for not checking crime.
Sourav Chowdhury was missing since Friday night, said his family. An empty cartridge shell was also found at the spot.
"We have filed a murder case. We are investigating the matter. I can't say anything more now," said North 24 Parganas' Superintendent of Police Tanmay Roy Chowdhury.
The body parts, lying scattered along the tracks between Duttapukur and Bamangachi stations, were discovered early Saturday.
Angry local residents set up road and railway blockades from time to time through the day demanding arrest of culprits. Police chased protestors with batons when the busy Jessore Road was blockaded.
The Bharatiya Janata Party (BJP) has called for a 12-hour shutdown at Bamangachi area in protest against the killing of the second year student. A state BJP team led by its chief Rahul Sinha will visit the area Sunday. Sourav's father Saroj Chowdhury is a local BJP leader.
The victim's brother said Sourav, who protested against anti-social activities in the Bamangachi area, was abducted from near his residence by some people late Friday night.
"From the balcony, I saw a group of people around 11 p.m.. When they passed by our house, I saw they were beating up and dragging my brother. I informed my father then. We informed the local panchayat member. But nobody came," said Sourav's brother.
A eye-witness gave more details about the ordeal that the youth had to undergo.
Opposition parties attacked the ruling Trinamool Congress for not checking crime
"I was returning after attending a marriage when I saw him being beaten up. They told me to get lost. I fell at their feet pleading that they release him. Seven of them were there. But in the darkness I could not recognize their faces. They also beat me up, and I had to leave."
The locals alleged Sourav had to pay with his life for protesting against illegal liquor dens and gambling rackets operating in the area.
"He used to protest against these rackets. So he was killed so cruelly by criminals," said a sobbing woman resident of Bamangachi.
State Education and Parliamentary Affairs minister Partha Chatterjee said he was trying to make enquiries to ascertain what had happened.
The BJP claimed that the victim and his family had close links with it, but state Food Minister Jyotipriya Mullick described Sourav as a Trinamool worker.
Communist Party of India-Marxist central committee member Mohammad Salim alleged that police have been kept under so much pressure by the ruling party, that when protestors are murdered, they are unable to ensure security.
"They cannot nab criminals. Gamblers are having a field day at the behest of the ruling party. On the other hand, police have been instructed to nip protests in the bud by inflicting torture on the people," he said.
State Congress president Adhir Ranjan Chowdhury accused the chief minister of "directly protecting" murderers and other anti-socials.
"It was the administration's duty to back Sourav. But everything is topsy-turvy here. Police and the administration are siding with gamblers and those running drinking liquor dens."
Incidentally, Barun Biswas - a crusader against rape - was murdered on this very day two years back by criminals in North 24 Parganas district.
On July 5, 2012, the 39-year-old school teacher was allegedly shot and murdered by the same criminals he protested against, at a railway station in Gobardanga block.
Lamenting Sourav's killing, Barun's elder brother Asit Biswas said: "I condemn this. But the scenario in our state is whoever protests faces arrest, or false cases are slapped against them".
Painter Samir Aich said: "I know very well that no step will be taken against the culprits. This is the way of the administration and government in this state".