At least 30 health workers were treated for gunshot injuries after security forces opened fire at a Wednesday protest, the nurses' union said, adding that four of them had to undergo surgery.

The demonstration took place as the kingdom, formerly known as Swaziland, shut down Internet access, making it difficult to gather information from across the country. Internet access remained cut off on Thursday.

"Nurses and other workers converged at the Coronation Park were met with unprecedented show of force by the police and the army," the Swaziland Democratic Nurses Union said in a statement.

"They were brutally dispersed and scattered all over the capital. As they were running, they were shot with live ammunition."

The 30 injured were among more than 80 reported hurt Wednesday in pro-democracy protests that have flared nationwide.

"Images that are coming from Eswatini are very disturbing indeed, and we can see that the political temperature is very hot indeed," Jeff Radebe, one of a group of mediators ordered to the country by the 16-nation Southern African Development Community, told the South African public broadcaster.

But he added that the "issues are very complex", saying the team was "going there with an open mind, ensuring that we hear all views, so that at the end of the day the people of Eswatini... come up with a lasting solution".

Demonstrations have run for more than two weeks, spearheaded by students, civil servants and transport workers.

King Mswati III is Africa's last absolute monarch, who enjoys flaunting his wealth and showering his 15 wives with lavish gifts.

Yet he rules over one of the poorest countries in the world, where nearly two-thirds of the population live in poverty and a quarter of adults have HIV.

- 29 dead -

In a statement, the Communist Party of Swaziland said the situation at the largest government hospital in Mbabane on Wednesday resembled a "war zone".

Hospital floors were "drenched in blood", said the party, adding that police "invaded the hospital, shooting even nurses as they attended to the injured, worsening the situation".

The union said security forces kept shooting at nurses into the evening, even as they were travelling to work night shifts at hospitals.

"Clearly these blood-thirsty imbeciles, brood of vipers are hellbent to kill nurses and the nation in defence of an ailing government," the union said, calling on its members not to treat any injured soldiers or police.

Five high school students arrested during protests were arraigned on terrorism charges Thursday over their role in the democracy push. Prosecutors accused them of burning down a police post.

At least 29 people have died since June in some of the worst unrest in the southern African country's history.