Swaziland's main pro-democracy group has asked the Commonwealth to impose sanctions on the king and his ruling elite, whom they accuse of running the country like a fiefdom.
The People's United Democratic Movement (PUDEMO) said in an open letter to the 53-nation grouping of mainly former British colonies that human rights were being suppressed in the tiny African kingdom.
Swaziland's young king, Mswati III, has drawn criticism at home and abroad for plans to buy a luxury jet and palaces for his 11 wives. Many of his subjects face hunger and the United Nations says they have the world's highest HIV/AIDS rate.
"PUDEMO believes that the Commonwealth has not acted strongly enough to intervene in the deteriorating conditions in Swaziland," it said in its letter, released on Monday.
"Indeed, the Commonwealth appears to have legitimised the drafting of a constitution which affirms the current power arrangements."
Commonwealth Secretary General Don McKinnon said last month that Swaziland was making satisfactory progress towards a return to constitutional rule after decades of absolute monarchy.
Mswati's father, King Sobhuza, tore up the constitution in 1973 and the country has been ruled ever since as an absolute monarchy where political parties are banned.
A new constitution, expected to be promulgated later this year, is supposed to guarantee civil liberties and other rights.
But critics say it leaves ultimate power in the king's hands and will do little to advance true democracy. Opposition parties will remain outlawed.
McKinnon said in June the Commonwealth was working with Mswati's administration on the constitution, and has dismissed the idea of imposing sanctions.
In its letter, PUDEMO asked the Commonwealth to consider imposing a travel ban on the ruling elite and freezing their assets in Commonwealth states.
"We believe that these resources are proceeds of corrupt practices and we recommend that they must be held in trust of the Swazi nation and handed over to a democratically elected government," it said.