The Foreign Correspondents' Club of Thailand (FCCT) had planned to host a panel discussion on the royal defamation law on Wednesday evening.
In a statement the FCCT said police had informed it that soldiers would blockade the building housing the club if the event went ahead.
"The FCCT has now been told that if the event goes ahead, the military will come and seal off access to the Maneeya Centre, where the club is located," it said.
The statement said police had conveyed the message on behalf of the National Council for the Restoration of Peace and Order.