The Communist Party of Swaziland said members attending a meeting with the South African high commissioner in Mbabane, found police waiting for them, armed with a typed list of their names.
Kenneth Kunene, the party's general secretary, told AFP the list was passed on by "someone from the commission".
"We furnished them with the names, so it is only them who had knowledge of who will be attending and when, and at what time," said Kunene.
There was some "physical and verbal confrontation" between the activists and police until the commissioner personally intervened and the meeting took place.
A staff member at the mission, which hires both locals and South Africans, is suspected to have colluded with the police and provided the list, he said.
"Some of them are Swazi. So possibly one has collaborated with the Swazi regime forces," said Kunene.
The South African deputy high commissioner Annell van Zyl confirmed to AFP that the police were at the offices but had not been asked to be there.
Asked about the clash with officers and the activists, she said "that I cannot deny or confirm, I really don't know".
Revealing the names of the mission's visitors was "not policy", said Van Zyl.
"Quite frankly if I should find out something like that there will be problems," she said, adding "it doesn't matter with whom we're talking...it's confidential".
"We don't prevent people from coming here to discuss or have dialogue in a friendly way. Not at all," she added.
Tensions have been running high in the mountainous land-locked kingdom since April 12, the 40th anniversary of a royal decree that transformed Swaziland from a democracy into an absolute monarchy.
"We hope that in the current situation in Swaziland diplomatic missions who receive visits from the pro-democracy movement do not collude in this way with the Swazi police," said Kunene.
"Our country is ruled by a brutal dictatorship, and missions from democratic countries should try to protect pro-democracy activists."
The activists handed over a memorandum that called for the condemnation of the arrests of dissidents in the small kingdom.
Last week they met with the United States embassy.