"We are one of China's most influential websites and we're not yet capable of our full duty to protect the country. For this we are even more regretful and ashamed," Sina said.
The web portal firm said it would deal with those responsible and introduce new mechanisms to deal with pornographic content. "We will handle and not avoid any serious punishment authorities impose on us," it said.
Shares in Nasdaq-listed Sina were down 7.3 percent at $49.37 at 1400 GMT.
The offending material was found after "a huge amount" of tip-offs from the public, Xinhua said. People suspected of criminal offences have been referred to police for investigation, it said without elaborating.
China's anti-pornography campaign was launched this month and officials say it will last until November. It comes after a crackdown on Wechat, Tencent Holdings Ltd's social messaging app, which has had dozens of widely read accounts run by outspoken columnists shut down.
It also follows a move last year to purge online rumour-mongering, widely seen by free speech advocates as a tool to punish critics of the ruling Communist Party. This has muted online demands from transparency campaigners, especially on Sina's Weibo, China's version of Twitter.
Analysts widely attributed a 9 percent decline in China's microblog users in 2013 to the crackdown.
© Thomson Reuters 2014
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