"Commemorating a terrorist organisation will not be allowed," police spokesman Ruwan Gunasekera said.
During the the Liberation Tigers of Tamil Eelam's 30-year campaign November 27 was celebrated as Heroes' Day (Maaveerar Naal) to mark the death of the outfit's first cadre Shankar in 1982.
In 1991, the week leading up to Maaveerar Naal was declared Heroes' Week. This resulted in slain LTTE leader Velupillai Prabhakaran's birthday on November 26 being included in the commemorations.
Responding to appearance of posters in Jaffna University premises calling for events marking Heroes' Day, Gunasekera said the police will take legal action against those involved.
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"I have information that there are plans to commemorate the day and re-visit the Eelam (a separate Tamil state) demand. It will be the government's duty to stop that," he said.
Rajapaksa accused the government of Maithripala Sirisena of going soft on LTTE supporters. He also criticised the release of some LTTE supporters by the government recently.
The LTTE battled Sri Lankan forces for three decades for a separate Tamil homeland. Rights groups claim government forces killed nearly 40,000 civilians in the final months of the brutal ethnic conflict.