South African trade unionists and Swaziland political activists have blockaded the countries' common border.
King Mswati is Africa's last absolute monarch
South African police say they arrested 25 people. Eight were injured when police opened fire.
The protests were against Swazi King Mswati III who is accused of ignoring human rights and political freedom.
Several hundred people demonstrated at five border posts. Police said they had fired rubber bullets to disperse the crowd at the Matsamo border post.
Congress of South African Trade Unions (Cosatu) spokesman Patrick Craven said all the injuries took place at Matsamo, bordering northern Swaziland.
Police said they had arrested 20 people at Matsamo, though Mr Craven said earlier that 46 had been arrested there.
The other five arrests were at Pongola, on the southern Swaziland border.
Police Superintendent Mtsholi Bhembe said the demonstrators had permission only to picket, and some had refused to move off the road they were blockading.
"It was at this point that we had to disperse the crowd with rubber bullets and arrest those who resisted moving from the road," he told the South African Press Association.
No demonstrations were reported on the Swaziland side of the border.
A spokesman for the organisers, the Swaziland Solidarity Network, said about 600 people had blocked traffic on the main route from KwaZulu-Natal used by the two countries' sugar industries, causing long tailbacks.
Demonstrators sang and carried placards at the Oshoek border post, the AFP news agency reports.
The spokesman said the group was calling for the establishment of a democratically elected constitutional forum and the legalisation of political parties.
"It is about time that we, as the workers of the region, throw our weight behind the people of Swaziland who are fighting for democratic reforms," Cosatu secretary general Zwelinzima Vavi told AFP.