BEIJING—The Chinese city of Chongqing was jolted by an unusual announcement: Its former police chief, who led a high-profile crackdown on crime that propelled local Communist Party leader Bo Xilai to national fame—and a possible top leadership position in Beijing—has gone on sick leave.

An announcement on the city government's microblog on Wednesday said that Wang Lijun was "receiving a vacation-style treatment." Mr. Wang was relieved of his police duties last week and given new responsibilities as a vice mayor of the southwestern metropolis.

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Wang Lijun before his sick leave, at a session last month of the Chinese People's Political Consultative Conference of the Chongqing Municipal Committee
The announcement followed rumors on China's Twitter-like microblogging site Weibo that Mr. Wang had sought asylum at the U.S. consulate in the city of Chengdu, also in southwest China. Photographs posted on Weibo showed several police cars outside the consulate late Tuesday night. A spokesman for the U.S. Embassy in Beijing on Wednesday declined to comment on the rumors, as did a spokesman for Chongqing's Public Security Bureau.

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Mr. Bo has been publicly silent on Mr. Wang, his close ally in a war on gangs that he launched after taking over as Chongqing's Communist Party Secretary in 2007.

The unexplained events ignited a storm of speculation on the Chinese Internet about the possible fallout of Mr. Bo, a charismatic figure who has carved out a unique position for himself as a populist politician in a country where politics is generally conducted behind closed doors.

Mr. Bo has attracted as much controversy as support by promoting a revival of Maoist values in his city. Among academics, journalists, diplomats and others in Beijing, a popular parlor game is guessing whether the politically ambitious Mr. Bo—the scion of a revolutionary family and a leading member of the so-called "princeling" faction of leaders ising in China—will make it into the all-powerful Politburo Standing Committee in a leadership reshuffle later this year.

Some believe that Mr. Bo has made himself politically vulnerable by appearing to run for high office like a Western politician, and that it is only a matter of time before he trips up. That calculation helps to explain the mounting national interest in the events unfolding in Chongqing.

The incident comes ahead of a high-profile trip to the U.S. by Chinese Vice President Xi Jinping. Mr. Xi is widely expected to replace Hu Jintao as president during the upcoming leadership transition.

The Chongqing government said that Mr. Wang had been overworked and was under undue mental stress in his new post as deputy mayor. News of his leave of absence quickly emerged Wednesday as one of the top topics on Sina Corp.'s Weibo microblogging service. Within a few hours, the announcement had been reposted more than 43,000 times.

Under Mr. Wang, police arrested dozens of underworld figures and seized weapons and drugs. The anticrime crusade was widely trumpeted in China's state-run media, though many lawyers have raised questions about the legal methods used, and have alleged Mr. Bo's campaign has demonstrated little regard for judicial procedure.

Many Chinese Internet users on Wednesday mocked the government's assertion that Mr. Wang had been placed on medical leave due to overwork.

"I'm offering free legal advice for 'sick people' seeking vacation-style treatments," wrote Li Zhuang, a prominent criminal-defense lawyer who was imprisoned in Chongqing for 18 months after defending an alleged gang leader in court.

—Olivia Geng contributed to this article.