Dumsile Khumalo, a junior police officer in the eastern town of Mliba, was at an inter-faith prayer service last week when she was heard speaking in a trance that a royal ritual now under way would be the last one, said Sidney Nyembe, secretary of Concerned Church, an association of Swazi religious leaders.
Senior religious leaders were all questioned by police and released, while Khumalo was suspended from her job, he said.
Khumalo was also ordered to appear before the head of royal affairs, Timothy Velabo Mtsetfwa, who will decide if she should be expelled from the police and exiled from her community for insulting King Mswati III, he added.
"We condemn this harassment in the strongest possible terms, because this fellow worshipper attended the prayer in her personal capacity and spoke the things she is accused of while in holy spirits," he said, referring to her trance.
"No one should be blamed or harassed for things said in prayer. We met her today... to encourage her and to tell her to be strong because she did nothing wrong. We then prayed for her," Nyembe told AFP.
Mswati is currently in seclusion for two months, a royal ritual observed annually that keeps him away from any official duties or family.
A financial crisis that has left government battling to pay salaries to civil servants has sparked a wave of pro-democracy protests this year, leaving the palace especially edgy about any criticism.