Sun Lin, better known as Jie Mu, and his partner He Fang were amongst 20 people apprehended from the cafe they operate in Nanjing on Tuesday.
Mu contributes reports to Boxun, a news website staffed by Chinese expatriates and nationals who submit online articles and essays on politics and human rights.
In recent weeks, he had published a series of online video reports investigating issues including media censorship and demolition of buildings against owners' wishes.
Boxun colleagues said Mu and the others had been arrested without warrants on claims of being mafia gangsters - but they claimed Mu himself had been chased away from the scene of one story by genuine mafioso. Boxun this morning reported Mu had been badly beaten after his arrest, after which his apartment was searched by police.
Reporters Sans Frontiers, the France-based group advocating freedom for journalists, called for Mu's release. It claimed government officials revoked Mu's press card in March and ordered him to stop reporting.
Mu turned to the web after the newspaper he founded, Da Du Shi, was apparently banned. Boxun, which is blocked to Chinese internet users, operates in both Chinese and English. The video reports he filed prior to his arrest were published at the international, English edition.
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