Police arrested senior trade unionists in Mbabane on Thursday before a rally against the hated tinkhundla regime, under which all political parties and activism are banned.
The deputy president of recently banned Trade Union Congress of Swaziland Sipho Kunene and deputy general secretary Muzi Mhlanga were among those arrested in the capital.
Police spokeswoman Wendy Hleta did not immediately respond to a request for comment.
Swazi trade unions had called for rallies to mark the 39th anniversary of the passage of a royal decree that effectively banned all dissent.
Prime Minister Sibusiso Barnabas Dlamini issued a statement on Wednesday saying security agents have been instructed to "protect life and property" and that the planned protests were illegal.
The National Union of Metalworkers of South Africa has issued an appeal to the "progressive and left-leaning governments, liberation movements of Africa, Latin America and Asia to support the struggle by the people of Swaziland to overthrow the current dictatorship of King Mswati and the royal family."
It called on South Africa's African National Congress government to act immediately to "isolate the illegitimate and unelected Swazi regime.
"The Swazi people are not alone in their quest to achieve their peace, freedom and democracy," the union declared in a statement.
"We will traverse the journey with them until King Mswati's regime is smashed and replaced with a truly democratic and people's government led by the democratic forces of Swaziland."