The leader, Iccha Gayen, was having lunch at a hotel close to the Kultali police station after the all-party meeting when he was gunned down, the sources said.
CPI(M) district secretary Sujan Chakraborty alleged that the killers belonged to the Trinamool Congress.
Sunderbans Development minister Shyamal Mondal denied the allegation and claimed it was the outcome of a CPI(M)-SUCI conflict in the area.
The minister said the killing would be inquired into.
No arrest was reported.
