The military told Bangkok-based Thai Lawyers for Human Rights in a written letter that if it has concerns about lack of freedom of expression or access to the justice system it should report them instead via a government hotline at the Interior Ministry.
The group condemned the army pressure, saying in a statement that it "creates the atmosphere of fear in society" and deprives people of their rights.
The May 22 coup deposed Thailand's elected government, and the junta that has ruled since has showed no tolerance for dissent.
Martial law is in effect, political assemblies of more than five people are banned, and the army has silenced hundreds of once-thriving political opponents, briefly detaining them and threatening jail terms if they speak out and disturb public order.
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Sutharee Wannasiri, a campaign coordinator for Amnesty International in Thailand, which was helping organize today's discussion, told The Associated Press that soldiers had phoned more than 30 times yesterday requesting the event be called off "because the situation is still not normal."
A statement issued by the lawyers group said the letter from the military was phrased as "a request for cooperation."
But officers said verbally that if the event went ahead, the group could face prosecution, according to the statement.
Thai Lawyers for Human Rights was set up about a week after the coup to monitor rights abuses and help provide legal advice to victims.