Calcutta: The 700-strong mob that hurled stones at cops and passers-by, set fire to buses and damaged cars on EM Bypass on Saturday were spurred into action by distrust in the system and a sense of immunity from police action, psychiatrists and psychologists said.

Cops allegedly remained mute onlookers for at least the first 90 minutes of the protest - violence followed by roadblock - that lasted for nearly three hours. They broke into a lathicharge after 1pm.

The mob had gone on the rampage after college students Biswajit Bhuniya and Sanjay Bonu were crushed under the wheels of a bus at the Chingrihata crossing on the Bypass around 11.15am.

Trouble erupted around 20 minutes later. They were displaying what experts described as "pent-up anger".

Psychiatrists said such mob violence is often an outcome of a collective grievance that "such incidents are happening, will happen again and unless we teach a lesson nothing will change" coupled with lack of faith in the system.

"Only when a group of people don't have faith in the system, will they take law in their own hands and try to deal with the problem in a violent manner because they feel violence is the best way to get the message across," said psychiatrist Jai Ranjan Ram, who had to change his plans for the evening to accommodate patients who were delayed by an hour or more because of the traffic snarl that followed the mob fury.

Sociologist Prasanta Ray pointed out that most members of a mob are "unrelated to the victims" and the cause of "mass discontent" is most often not directly related to the incident.

"There is a certain relief or a sense of satisfaction derived from engaging in violence. The mob also enjoys a sense of immunity because they are not traceable," he said.

Ray said that if "the reason for mass discontent can be removed, it might reduce the anger level".

Ram said grief or anger results in tunnel vision. Hundreds of people suffered because of the roadblock, violence and subsequent snarl for no fault of theirs but that did not matter to the mob.

Mob violence frees an individual from a sense of guilt in indulging in something s/he should not do, psychiatrists said.