Nkolisi Ngcamphalana, national organiser for the youth wing of the banned opposition People’s United Democratic Movement (PUDEMO), told AFP that he was detained Tuesday night and held for 24 hours.
He said police beat him, placed plastic bags over his head, and then interrogated him about calls for national protests on Tuesday.
“They grilled me about the April 12 uprising, and they kicked and beat me,” he said shortly after his release.
“Right now they are outside my house. They are intimidating my family,” he said. “They threatened me with death.”
Ngcamphalana said he would go into hiding as police had threatened to charge him under the Suppression of Terrorism Act.
Police did not immediately reply to requests for comment.
Several other activists have been arrested as police raided their homes in recent days, said Maxwell Dlamini, head of the Swaziland National Union of Students which has led the protest calls.
“We expect more raids in the coming days,” he said.
Swazi authorities were tightening security across the country, with soldiers searching cars at the border with South Africa and police setting up roadblocks along the highway.
Mswati, 42, who has ruled the impoverished kingdom since he was 18, is accused of bankrupting the state coffers with his lavish lifestyle and 13 wives.
Efforts to slash salaries of civil servants sparked the biggest protests seen in years on March 18, as labour unions rallied workers into the streets.