China's online population -- the largest in the world -- has topped half a billion, the state Xinhua news agency reported on Thursday, citing a senior official. Wang Chen, chief of the information office of China's State Council, or cabinet, said more than 15 million people had gone online since the last official figures were released in July. The growing strength and influence of the web population has fuelled concern in Beijing about the Internet's potential as a tool for generating social unrest, and authorities have stepped up surveillance in recent years. The government blocks web content that it deems politically sensitive in a vast system dubbed the "Great Firewall of China". But it has struggled to control the increasingly popular weibos, or microblogs -- sites similar to Twitter that provide a new avenue for mass expression in a tightly controlled information landscape. Speaking at an industry roundtable, Wang said China now had more 300 million microbloggers, while the number of rural rural Internet users had reached 130 million.