Pro-democracy activists held protests earlier this month
Police in Swaziland have detained a number of pro-democracy activists planning a border blockade ahead of parliamentary elections in the kingdom.
Several union leaders were bundled into police vans at the main border crossing with South Africa, organisers of the planned blockade said.
Political parties are banned in Swaziland, one of the world's last absolute monarchies.
There have been recent protests calling for change and multi-party democracy.
In Friday's elections, 55 unaffiliated candidates are due to be chosen for Swaziland's national assembly.
King Mswati III, who has been in power since 1986 and recently celebrated his 40th birthday, hand-picks a further 10 people.
The assembly then chooses 10 representatives for the Senate, with a further 20 selected by the king.
Correspondents say the king remains popular with many of his subjects, though opposition has been growing to his lavish lifestyle in a country where most people live in poverty.
Early on Thursday, authorities detained union leaders and other activists after they arrived at Oshoek, Swaziland's main point of trade with South Africa.
Among those being held was Jan Sithole, the secretary-general of the Swaziland Federation of Trade Unions, who said he had been driven about 120km from the border to Piggs Peak.
Other would-be protesters in buses and cars were turned back by Swazi authorities.
The BBC's Thulani Mthewa, who is at the border, said some 200 people - both South African and Swazi - protested on the South African side of the crossing, but the demonstration on the Swazi side had been blocked and the border remained open.
South African unionists from the umbrella organisation Cosatu are demonstrating in solidarity with their Swazi counterparts.
"The problem of Swaziland is our problem because the current regime is not resolving many problems - HIV/Aids, unemployment - their people are flocking in our country," said Kesco Mataba of the South African Young Communist League.
"The whole world was making noises about Zimbabwe," he said. "It's the same problem with Swaziland."
Riot police have been deployed on both sides of the border crossing, our reporter says.
Landlocked Swaziland is almost entirely surrounded by South Africa.
Ahead of the planned blockade, Swazi government spokesman Percy Simelane said the protest was illegal.
"No one is allowed to wake up in the morning and go and close borders leading to another country, not even the government is allowed to do that," he told the BBC's Focus on Africa programme.
But the secretary-general of the Swaziland Federation of Labour, Vincent Ncongwane, said protesters wanted to demonstrate that Friday's elections would not be inclusive.
"We still have in Swaziland this myth that you can have a democracy where there isn't the participation of other political parties," he told the BBC.