Outside of the square, uniformed and plainclothes police kept watch on the streets in southern Africa's usually peaceful tiny mountain kingdom.
In recent weeks, an online campaign has tried to rally support for Tuesday's protests, which come exactly 38 years after the current Swazi king's father, King Sobhuza II, banned political parties and abandoned the country's constitution.
Two reporters for a South African radio station were released on Tuesday after being detained for a few hours in Swaziland, where they were sent to cover the planned protests.
Talk Radio 702's report said police escorted the two reporters out of Swaziland's commercial capital. News of the reporters' arrest followed a statement by a pro-democracy group in Swaziland that five activists were arrested ahead of planned mass protests.
Observers have reported police putting up road blocks throughout the country in recent days.
An anti-monarchy movement has gained momentum since the government declared a budget crisis and proposed freezing civil service wages. But many Swazis revere the monarchy.