Swaziland's main trade union has begun a two-day general strike to press for democratic reforms.
King Mswati is often criticised for his lavish spending
Union leader Jan Sithole told the BBC that roadblocks mounted by the security forces had stopped people from joining the protests in the capital, Mbabane.
The unions say a draft constitution being debated by parliament entrenches the power of the monarchy.
Swaziland is Africa's last remaining absolute monarchy and political parties are banned.
King Mswati III, 36, is often criticised for having 11 wives and his lavish spending, while most Swazis live in poverty and more than 30% are HIV positive.
The police have given permission for the demonstrations but they fear an outbreak of looting.
According to the BBC's Thulani Mthethwa in Mbabane, the capital resembles a military zone with water tankers and armed vehicles patrolling the city.
Buses carrying workers into the city centre are being thoroughly searched and the drivers must have permits before being allowed to proceed.
Mr Sithole, leader of the Swaziland Federation of Trade Unions, had said he had expected not less than 20,000 workers to attend the protest, but our reporter says it is not likely to be more than 5,000, as teachers and civil servants are not participating.
Although most Swazi support the strike's sentiments, they feel striking is no longer an effective way of mounting pressure on the government, favouring blocking the border with neighbouring South Africa instead, our correspondent says.
The government says that most Swazis prefer a traditional form of government.
But Mr Sithole said the new constitution should guarantee the separation of powers between various branches of government.
"We would ensure that we entrench the institution of the monarch into the constitution so that we have a constitutional monarch," he told the BBC's Network Africa programme.
The union was also fighting for women's rights and more respect for human rights in general, he said.
He hoped the king would now not give the new constitution his ascent and would remember his vow to be ruled by his subjects.
"He will be put to the test to listen to his people because we are saying this constitutional process has been flawed."