Police in eSwatini - formerly known as Swaziland - have arrested the head of the southern African country's largest opposition party and several other politicians and activists in raids on their houses, their parties said.

The operations on Friday took place as people's anger against King Mswati III, an absolute monarch, has swelled in the past year.

Police Commissioner William Dlamini told reporters the police had "invited some individuals to assist us in our enquiries pertaining to state security information", but denied that any of them been arrested or detained.

Spates of anti-monarchy demonstrations have taken place across eSwatini this year. Some have turned violent with police using tear gas, stun grenades and water cannon to disperse stone-throwing protesters.

Demonstrators say the king drains public coffers to fund a lavish lifestyle while most of his 1.5 million subjects eke out a living toiling in maize or sugarcane fields.

A decade ago, Forbes estimated Mswati's net worth at $200m, a figure he has said was made up.

Percy Simelane, a spokesman for the government and the king, referred Reuters News Agency to the police when asked for comment.

Some of the protest marches were organised by the Political Parties Assembly, whose leaders have been targeted in the raids.

Mlungisi Makhanya, president of the People's United Democratic Movement (PUDEMO), told Reuters that the party's secretary-general Wandile Dludlu had been arrested and political materials and electronic devices had been seized.

He added that a legislator and former trade union leader and the presidents of the Swaziland National Association of Teachers and the Ngwane National Liberatory Congress had also been targeted.

The Communist Party of Swaziland said its national chairperson, Dumisani Fakudze, had also been detained.

Trade unionist Jan Sithole told Reuters after being released that the police had kept his mobile phone, laptop and flash drives.