Censors were stationed for a week at the offices of Tencent's popular social media service WeChat before dozens of prominent accounts were closed or suspended, a source has said.
The move, on the closing day of the National People's Congress, marked an intensification of the internet crackdown under President Xi Jinping's leadership, observers said.
Internet users noticed on Thursday evening that at least 35 WeChat public accounts, many known for carrying commentaries on current affairs, were no longer available due to what a system message said was "violation of regulations".
A source told the South China Morning Post that a team of internet censors had been sent to WeChat's Guangzhou office, where they remained for about a week before the crackdown.
The Beijing-based industry insider, who asked not to be named, said the censors asked WeChat to conduct self-censorship in relation to popular public accounts that provided sensitive content on national politics, while they also named some accounts to be shut down.
"It might be related to some sensitive articles published by Tencent recently," the source said. "It feels like a punishment of Tencent, and we expect the censorship on WeChat will be stricter."
Zhang Lifan, a Beijing-based political affairs analyst, said: "Of course the shutdown is ordered from above.Tencent is a victim too."
Zhang said some account holders may have been tipped off about the crackdown, as he noticed they had stopped posting sensitive content two or three days ago.
"The closing day of important meetings seems to have become a regular time to clamp down on freedom of expression," said Zhang, whose WeChat public account, Sina Weibo account and blogs were shut in November on the day the party Central Committee's third plenum ended.
Another account holder affected is Xu Xin, a law professor at Beijing Technology University. Xu, who had more than 30,000 subscribers, said the move was unwarranted. His recent postings were only about films and law. "Which one of my article is sensitive? Which law did I violate? Who reported me? Why can't I appeal?" he asked.
Owners of affected accounts and their followers have been quick to turn to alternative platforms. Xu said he had since authorised Lawside, another WeChat public account to publish his articles "at [their] own risk".
Xu Danei , whose WeChat account, with an estimated 200,000 subscribers, was also shut, encouraged his followers to turn to a content-delivery app he had developed.
The South China Morning Post reported that foreign media were warned ahead of Premier Li Keqiang's press conference on Thursday not to raise the graft investigation into ex-security tsar Zhou Yongkang .
"The warning and the WeChat crackdown on the same day may be linked", Zhang said. "[The crackdown] is possibly a pre-emptive muting as part of a round of intensified censorship as the party prepares to make the announcement [of Zhou's fate]."
Tencent's investor relations director, Yuntao Huang, did not comment on the reasons for the crackdown.
Additional reporting by Angela Meng