According to colleagues, Peter Jesper Dahlin was detained "on suspicion of endangering state security" on January 4.
"A Swedish citizen in his mid-30s has been detained in China," a spokesman for the Swedish Embassy in Beijing told CNN, adding that consular staff were currently investigating the matter.
Dahlin worked for the Chinese Urgent Action Group, which "undertakes rapid response assistance for rights defenders in need" and provides legal aid and training across China, according to its website.
The Chinese Ministry of Foreign Affairs did not respond to a request for comment.
Dahlin's detention comes as activists say Chinese authorities are ramping up their campaign against legal aid and human rights workers.
Crackdown on lawyers
This week, five Chinese human rights lawyers were charged after being held in secret for six months, supporters said.
Zhou Shifeng, founder of a Beijing law firm that has been at the heart of the ongoing crackdown, is accused of "state subversion," colleague Liu Xiaoyuan said on his verified Weibo social media account Tuesday.
The charge carries a potential maximum sentence of life in prison.
Four other lawyers were charged with "incitement to state subversion," which carries a maximum sentence of 15 years, according to notices posted on social media by supporters.
The crackdown, which has been ongoing since at least July last year, has seen dozens of lawyers, activists and their relatives taken into custody or questioned by police in cities across China
In December, outspoken human rights lawyer Pu Zhiqiang went on trial in Beijing
, accused of "picking quarrels" and "inciting ethnic hatred" based on seven Weibo posts between July 2011 and May 2014.
"Lawyers and civil society leaders such as Mr. Pu should not be subject to continuing repression, but should be allowed to contribute to the building of a prosperous and stable China," the U.S. Embassy in Beijing said in a statement at the time.
Hong Kong connection
Dahlin is not the only Swedish citizen to get caught up in China's crackdown on civil society groups.
Publisher Gui Minhai, who holds a Swedish passport, is one of five Hong Kong men involved in the publication of books that are critical of or embarrassing to Chinese leaders that has disappeared in recent months.
The latest to vanish, Lee Bo, disappeared last month. His company, Causeway Bay Books, was due to publish a book on Chinese president Xi Jinping's alleged love affairs before he came to power, according to Hong Kong pro-democracy lawmaker Albert Ho.