Six hours into the back-to-back "24" marathon, Jack Bauer was foiling Russian assassins, trying to save the world from nuclear catastrophe -- and holding a Chinese audience spellbound.

"It's really great; I've seen every season," said university student Niu Dajun, 23, one of about 100 Red Bull-fuelled watchers at an all-night viewing in a Beijing cinema.

"American TV shows are really innovative. Chinese ones tend to be more conservative. That's one reason why US series are so appealing to us."

Chinese viewers have a voracious appetite for Western television series, with demand soaring as more and more young people turn towards online video and away from domestic TV offerings they consider lacklustre.

But official censors late last month abruptly took four hit US TV series offline, including the hugely popular "Big Bang Theory", which had garnered more than 1.3 billion views on the Sohu online video site since 2009.

Observers say the crackdown is less a matter of inappropriate content than a move by Communist authorities to assert their control over a newly flourishing domain.

The first two episodes of the new ninth season of "24" were released this week on Sohu's rival Youku Tudou, which organised the cinema event, and ranked among its most popular programmes this week.

- Is Sherlock gay? -

Western TV series began gaining mass appeal in China about a decade ago with the rise of pirate DVD versions of US programmes such as the NBC sitcom "Friends" and HBO's "Sex and the City", Chinese media analysts say.

Online streaming sites such as Youku, Sohu and Tencent's QQ Video are now delivering a deluge of advertising-supported legal content to China's 618 million Internet users.

The Netflix drama "House of Cards", a dark tale of US political power that also features a Chinese subplot, has gained a national following reportedly including Wang Qishan, a member of the seven-strong Politburo Standing Committee, China's most powerful body.

The BBC detective series "Sherlock" series is keenly followed -- with mass speculation that the title character and his sidekick Watson are a gay couple. When British Prime Minister David Cameron visited last year, Chinese Internet users' most popular question for him was when the next season was starting.

But the success of such programmes has also drawn the attention of authorities, who previously turned a blind eye to online video amid a turf war over which arm of the Communist Party bureaucracy was responsible for its oversight, according to industry experts.

"These American online TV shows, this is the year when they reached a point of popularity at which the regulators started to take a look at them and do what they usually do, which is stamp out and regulate," said Jeremy Goldkorn, founder of Danwei, a Beijing-based firm that tracks Chinese media and the Internet.

"It sort of feels like this is a first step," he added. "What will be next is unlikely to be liberal."

The four US shows pulled off Chinese video sites at the end of April -- "The Big Bang Theory", "The Good Wife", "NCIS" and "The Practice" -- explore topics ranging from the US legal system to the lives of a group of geeky California scientists.

Commercial interests may also be at play, with reports saying China's state broadcaster CCTV will soon show a "cleaned-up" version of "The Big Bang Theory".

- 'Compulsively censorial state' -

Western TV series are able to delve into territory deemed off-limits for Chinese programme makers, said Ying Zhu, a New York-based expert on Chinese media.

Beijing's powerful State Administration of Press, Publication, Radio, Film and Television has never published a list of what can or cannot be broached on Chinese television.

But family dramas, game shows and epics about China's struggle against Japan in World War II are typical offerings. Contemporary political dramas, corrupt officials or content that might be deemed morally "unhealthy" are conspicuous by their absence from the screens.

"Under the thumb of a compulsively censorial state, China's own entertainment industry is not allowed to make use of its own best material," said Zhu.

While Western programmes that succeed in China typically have high production values, Goldkorn noted, they also have "the ability to take on themes of social disharmony of one kind or another, which is really what makes most good theatre work".

At the "24" event in Beijing, viewers hoped the four recently blocked US series would go back online soon.

Cheng Lu, a 25-year-old bank employee, vented her frustrations that "almost all Chinese shows have the same plot", and ticked off some of the benefits of US programmes.

"They're good for helping you increase your English listening comprehension," she said. "Also, you can watch cute guys."