BEIJING (Reuters) - Chinese police threatened a muck-raking investigative reporter on Monday with arrest for concealing evidence if he did not hand over further tapes in his possession after a sex video he released of an official with a mistress went viral online.

Zhu Ruifeng, who runs a whistleblowing website called "People Supervision Net", released the video last year of Lei Zhengfu, a district party chief in the southwestern city of Chongqing, having sex with his much younger mistress.

Communist Party officials are banned from having mistresses, and the video came to symbolize in many people's eyes the excesses and corruption of the ruling elite, which the government is trying to stamp out.

Lei was fired shortly after the graphic images were splashed across microblog websites and the case was widely discussed in state media.

Zhu subsequently said he had sex tapes of other officials and that he would also release them.

That prompted a visit to his home in Beijing on Sunday night by police from Chongqing, Zhu's lawyer Li Heping told Reuters.

Zhu eventually agreed to go with them to a police station on Monday to answer questions.

"The police want him to hand over the other tapes," Li said by telephone. "If he does not, they are threatening to charge him with being a witness and concealing evidence.

"As one of his lawyers, we believe that he has the right to protect his sources. If sources are threatened, then they will no longer be willing to hand clues over," he added.

Zhu was released later in the day. He did not answer his telephone on Monday.

The case underscores the government's quandary in fighting graft and allowing enthusiastic outsiders to get involved and exercise independent oversight.

While the government has been keen to encourage the country's hundreds of millions of Internet users to expose graft online, it remains extremely wary of giving people too much free rein, and senior officials remain off limits.

Chongqing is especially sensitive because it was the power-base of now-disgraced former top leader Bo Xilai, who was dramatically sacked last year amid lurid allegations of corruption and murder.

While the party has stepped up its rhetoric against corruption, seeking to counter anger from citizens over regular reports of graft and debauchery among officials, efforts to root out the problem have hardly made a dent.

(Reporting by Ben Blanchard; Editing by Robert Birsel)