* Sina crackdown seen as a warning to Internet firms
* First time anti-pornography body has penalised web firm -expert
* Move seen as power play as govt bodies clamour for authority
By Paul Carsten and Michael Martina
BEIJING, April 25 (Reuters) - A move by China's authorities to strip Sina Corp of some online publication licences as part of a pornography crackdown should be seen as a warning shot at the country's Internet firms to toe the Party line on freedom of expression, experts said.
The punishment was also initiated by a regulatory body that has previously not exercised its power in this way, and is being widely viewed as a sign that various branches of the government are clamouring to assert their authority over the Internet.
The revocation of Sina's licences for online literature, video and audio publication is the harshest in recent memory for a large Chinese Internet company. But it will inflict minimal damage to the company which gets most of its money from advertising on its more popular websites such as those for news, sport and entertainment.
"It's almost a symbolic move, they're targeting Sina, which is a big name, because they know everyone will write headlines about it, but it won't have an impact on the company's revenues or profits," said Doug Young, a professor at the Fudan University Journalism School.
"But at the same time it sends a message out to companies, 'Hey, you guys need to be watching your sites and actively policing it to avoid sensitive content and remove it'," he said, adding that he did not think other firms would be targeted now that the message was out.
The licences were revoked after twenty articles and four videos posted on Sina.com were found by the National Office Against Pornographic and Illegal Publications to contain "lewd and pornographic content", the official Xinhua news agency said on Thursday.
Sina issued a statement that it was sincerely apologetic and that it is now closely communicating and cooperating with authorities. It is unclear if or when Sina will get its licences reinstated.
Shares in Sina, which trades on the Nasdaq, slid more than 7 percent but later pared losses to end 3 percent lower.
The move came amid an anti-pornography campaign that was launched this month and which officials say will last until November.
But it is also part of a wider crackdown on online freedom of expression that has intensified after President Xi Jinping came to power early last year, and which has drawn criticism from rights groups and dissidents both at home and abroad.
Tencent Holdings Ltd's social messaging app, WeChat, was also targeted last month, with dozens of widely read public user accounts run by outspoken columnists shut down. Last year, there was a move to purge online rumour-mongering which was seen by free speech advocates as a tool to punish critics of the ruling Communist Party.
New centres of power created under Xi have likely caused regulators with varying degrees of authority over the Internet to jockey for position and recognition.
"There's a bit of competition for who's in charge of the Internet, so people have to be seen to be getting notches in their belt," said Duncan Clark, chairman of BDA, a Beijing-based tech advisory.
"To retain your power and not lose your power you need to be seen to enforce. What this does is make people nervous: who's next? It seems like there's change in the air."
Zhan Jiang, a journalism professor at the Beijing Foreign Studies University, also argues that the move by the National Office Against Pornographic and Illegal Publications was one with little legal justification.
"If Sina are guilty of such behaviour it should be punished by the Ministry of Industry and Information Technology or the State Council Information Office," he said.
According to Zhan, this is the first time an Internet company has been punished by the office, which usually targets traditional media, and Sina's publication of pornography isn't unique to China's large Internet firms. (Additional reporting by Megha Rajagopalan and Beijing Newsroom; Editing by Edwina Gibbs)