A UN agency protested Thailand's deportation of two registered refugees to China on Wednesday, saying they should not have been sent back to a country where their lives could be in danger.
The United Nations High Commissioner for Refugees agency said the two individuals were in possession of a UN "protection letter".
They were waiting to go to Canada, having been accepted as refugees, at the time of their arrest by Thai authorities.
"These people are recognised refugees, meaning they were interviewed and their claim of persecution was found to be legitimate," Vivian Tan, regional spokeswoman for UNHCR, told Reuters.
"They should not be sent back to a place where their lives can be put in danger."
The refugee agency did not name the activists or their nationalities, but an official at an Immigration Detention Centre in Bangkok gave details of their case.
Jiang Yefei and Dong Guangping were arrested on Oct 28 following a request from China, the official, who declined to be named because of the sensitivity of the matter, told Reuters.
Chinese Foreign Ministry spokesman Hong Lei said the issue was being handled "in accordance with the law". He did not elaborate.
The Canadian embassy in Bangkok did not offer any immediate response to a Reuters request for comment.
Thailand's generals have cultivated warmer ties with China since seizing power in a 2014 coup. The coup was widely condemned by Western nations, which downgraded diplomatic ties, but the ruling junta claimed to have support from China.
Thailand has not signed a 1951 Geneva Convention relating to the Status of Refugees, nor does it have a specific law on refugees.
"By sending the men back to China where they could face torture, Thailand is repeating its bad record on refugees," said Sunai Phasuk, Thailand researcher at Human Rights Watch.
Thailand deported about 100 Uighur Muslims back to China in July, drawing condemnation from the United States and others.
The Uighurs are a Turkic-language speaking group that calls China's western Xinjiang region home.
Thai immigration officials said the two deported men were not Uighurs.
According to the pro-dissident online group Front Line Defenders, which is based in Ireland, Mr Jiang and Mr Dong were taken from their Bangkok residences and placed into custody on Oct 28 by Thai immigration police.
They were held under police watch at Mr Jiang's home until they were taken to the airport for rendition to China.
The group said Mr Dong, his wife and their child had arrived in Thailand in late September to apply for asylum with the UNHCR in Bangkok. The family received UNHCR "letters of protection", supposedly good while their cases were being considered.
Mr Jiang, meanwhile, had been in Thailand since 2008. He received the same refugee status from the UNHCR as the Dong family last April. According to Front Line Defenders, he is chairman of the Thai branch of the Federation for a Democratic China.
A statement by Front Line Defenders said the group "believes that their arrests solely result from their peaceful and legitimate activities in the defence of human rights in China, and that their detentions may have been orchestrated by the authorities in China".
Radio Free Asia said Mr Jiang is a political cartoonist who was detained and tortured after criticising China's ruling Communist Party for mishandling relief and recovery efforts after the massive Sichuan earthquake in 2008. It said he has been living in Thailand since fleeing his country, and was granted refugee status by UNHCR in April.
Mr Dong was imprisoned for subversion in 2001-2004 and was secretly detained for eight months in 2014 before fleeing China with his family two months ago, RFA said.