Peter Dahlin, a Swedish human rights activist, was detained on national security charges in China.
China has released a Swedish man taken into custody this month on suspicion of acts detrimental to the country's national security, the Swedish Foreign Ministry says.
China has drawn international condemnation over a crackdown on rights lawyers. Three weeks ago, it detained Peter Dahlin, a 35-year-old co-founder of the Chinese Urgent Action Working Group that worked with Chinese human rights lawyers.
Dahlin was expelled from China on Monday and had departed for Sweden, Michael Caster, a spokesman for his group, said in a statement, adding that Dahlin's girlfriend, Pan Jinling, who had been detained around the same time, had also been released.
The Swedish foreign ministry said it remained concerned about naturalised Swedish citizen Gui Minhai, a Hong Kong-based bookseller who had vanished in October in Thailand.
Gui appeared on Chinese state television this month, saying he had surrendered to authorities over a fatal drink-driving offence more than a decade ago.
The disappearance of Gui and other booksellers has prompted fears that mainland Chinese authorities may be using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.
In a statement, Sweden's Foreign Minister Margot Wallstrom said she welcomed Dahlin's release, but expressed concern over Gui.
"Meanwhile, I am greatly concerned over the detained Swedish citizen Gui Minhai," Wallstrom said, adding that Sweden was working to get clarity on his situation and an opportunity to visit him.
China's ministry of foreign affairs did not immediately respond to a request for comment.