"This is a national crisis," said Jan Sithole, secretary-general of the Swaziland Federation of Trade Unions.
But the royal family sees it as a simple case of an impertinent woman with no respect for tradition in a nation where the king is above the law.
"We have traditional forums where such issues should be addressed and we are surprised this woman decided to go to court," said Malamlela Magagula, a royal adviser. "The (king) wondered whether (the) mother was really a Swazi citizen or not."
The conflict began after Mswati's aides picked three women he spotted at the annual reed dance festival and took them to secret locations to become his newest wives.
Though many consider it an honor to have a child chosen to be a royal bride, Lindiwe Dlamini was horrified that her 18-year-old daughter Zena Zoraya Mahlangu was taken and decided to sue.
Since the case began last month, the royal family has repeatedly ignored the court, refusing on several occasions to let two women appointed by the court interview Mahlangu to determine if she wants to marry the king.
Dlamini also has not been allowed to contact her daughter.
Last week, Attorney-General Phesheya Dlamini, the national security chief, police commissioner and army commander visited Chief Justice Stanley Sapire and the two other judges hearing the case and demanded they either dismiss the lawsuit or resign.
On Sunday, Mswati, 34, became engaged to Mahlangu, a step that all but sealed her fate to become his 10th wife.
The High Court still plans a hearing Tuesday on the lawsuit.
Trade unionists, opposition groups, civic officials, lawyers and ordinary Swazis condemned the royal family's actions.
"Their conduct blatantly undermines the independence of the judiciary and directly interferes with the smooth administration of justice and the rule of law," the Law Society of Swaziland said.
Rights activists have condemned the king's policy of taking brides without asking the women's permission or talking to their families.
"Such a practice, Your Majesty, is degrading, dehumanizing and traumatic to the dignity and person of women folk," Vulindlela Msibi, president of the Human Rights Association of Swaziland, wrote in a letter to Mswati.
Amnesty International said the royal family's actions followed a long-standing pattern of discrimination against women in the southern African country of 1 million people.
"The king and his agents have violated the internationally recognized human rights of women and girls, including their right not to be arbitrarily detained and the right not to be subjected to forced marriage," the human rights group said.
Mswati can marry as often as he pleases. His father, King Sobuza II, who died in 1982, had more than 100 wives.
Many Swazis have been annoyed with Mswati since last year, when the king banned girls under 18 from having sex — a decree he said was intended to halt the spread of HIV.
A few weeks after declaring the ban, Mswati took a 17-year-old girl as his ninth wife. Eventually, he paid a fine of one cow.
Mswati also came under fire for a decree he issued last year that stripped the courts of their independence and muzzled the press. Under international pressure, he revoked it.
Swaziland won independence in 1968 from Britain, which left it with a democratic constitution establishing a constitutional monarchy and a bill of rights. But in 1973, a year after the pro-royalist ruling party won elections, Sobuza repealed the constitution.
"Since then, Swaziland has never known anything that can be done against the will of the king," said Bongani Masuku, secretary-general of the Swaziland Solidarity Network, which is calling for democratic reforms.
Though Swaziland has a court system, a Parliament and a prime minister, all answer to the king in some way, said Sithole, the union leader.
Masuku and other opposition leaders complain that Mswati has made himself the final arbiter of Swazi traditions.
"People must passively and submissively accept everything he does as Swazi law and custom," Masuku said. "Anything he likes is Swazi custom. Anything he dislikes is not Swazi custom."