A high-profile Chinese politician whose rapid rise has been placed in doubt by the detention of a close aide on Friday denied he was himself under investigation, as speculation about his future intensified.

Bo Xilai, the charismatic but controversial Communist Party secretary of Chongqing, has been the subject of intense speculation since the police official who orchestrated his high-profile crackdown on corruption was detained.

The official, Wang Lijun, spent a day at a US consulate in China, where he reportedly held discussions about defecting before he was taken into police custody last month and flown to Beijing.

The political drama has fuelled speculation about Bo's career prospects, with analysts saying the saga could end his ambitious hopes of joining China's top-decision making body later this year, when a major power transition begins.

On Friday, Bo sought to downplay the significance of the incident, telling a news conference on the sidelines of China's annual parliamentary session that it was an isolated case.

He said the investigation into Wang, details of which have been kept under wraps, related to the fight against organised crime in the southwestern mega-city of Chongqing.

China's popular social networking sites buzzed with rumours on Thursday that "something big is going to happen" after Bo failed to appear at a parliamentary session earlier in the day that was supposed to be attended by all lawmakers.

Thursday's main evening news bulletin added fuel to the speculation after Bo was shown on television after the lower-ranked Chongqing mayor Huang Qifan.

Chinese leaders typically appear on television in the order of their rank in the political hierarchy, with the most senior leaders appearing first.

But Bo told reporters Friday he had missed the political session because he had a cough and was not feeling well, and apologised for the fuss he had caused.

Bo made his reputation with a crackdown on corruption and crime in Chongqing, but some critics charge he relied on unfair convictions and neglected due process.

His campaign to revive Communist culture by promoting "red" songs and revolutionary slogans has also divided opinion.

Wang, vice mayor and former police chief of Chongqing, is famed as one of China's top graft-busters after leading the crackdown that led to scores of senior officials being jailed in the city of 30 million people.

A vacant seat at the news conference in Beijing's Great Hall of the People on Friday, which was occupied by Wang last year, had a sign saying "absent for holidays", according to Hong Kong's Phoenix TV.

The political intrigue surrounding Wang deepened Thursday after Chongqing property developer Zhang Mingyu was detained in Beijing after he threatened to publicly explain "Wang Lijun's jigsaw puzzle", his lawyer told AFP.