Four supporters of Bengal's Left Front ally, the Forward Bloc (FB), were killed in police firing on a demonstration in Dinhata town in Coochbehar district on Tuesday.
Two persons died at the venue of the agitation near the Dinhata SDO's office while two more succumbed to their injuries later.
A furious FB has called a 24-hour Bengal bandh on Wednesday, making this the first time in the history of the Left Front in Bengal that a ruling coalition partner has called a general strike against the state government.
Forty persons, including 25 police personnel were injured in the violent clashes and subsequent police firing.
Coochbehar SP Anil Kumar told HT that firing was resorted to as the demonstrators had gone berserk, torching police and official vehicles and ransacking state government offices and damaging property after they were prevented from proceeding to the SDO's office.
Coochbehar District Magistrate Rajesh Sinha confirmed that four persons had died.
The firing was preceded by a lathicharge and teargas shelling, which police officials said did not suffice to quell the agitators' wrath.
The FB's Coochbehar district committee secretary Udayan Guha however said the agitation was peaceful and the police firing was unprovoked and done at the behest of the CPM.
He also dismissed the allegation that his party supporters had gone berserk, prompting the police to open fire on them.
After Tuesday's firing, furious FB leaders said the death of four party members was a slap on the face of the Left's unity.
Political differences between the FB and CPM had come out into the open after the violence in Nandigram. But after Tuesday's violence, the rift has clearly got deeper and more bitter.
The junior Left partner has already decided to separately fight the panchayat elections in the state in April. The FB has also put up 13 candidates against the CPM in the 60-member legislative assembly of Tripura, which goes to the polls on February 23.
Currently, four ministers in Bengal belong to the FB and after the CPM, it is the second largest Left party in the state, with 23 MLAs.
"There was no provocation (to fire on the agitators). The rally at Dinhata was part of a statewide civil disobedience programme. The administration had been informed in advance. The killings are a slap on Left unity,'' FB general secretary Debabrata Biswas told HT.
FB secretary G Devrajan said the violence was an attempt to suppress a democratic movement. "The participants at the rally were peaceful and were protesting against the price rise, retail trade, setting up of special economic zones and demanding implementation of the Sachar Committee report,'' Devrajan said.
Whether the FB would finally want to quit the Left front at the national level remains to be seen. At the national level, the CPM has 44 MPs whereas the FB has only three. As an independent entity, quite clearly it will not be able to enjoy the clout it currently does as part of the four-party Communist-led front.