China has unveiled tough measures to stop the spread of what the government calls irresponsible rumours, threatening offenders with three years in jail if untrue posts online are widely reposted, drawing an angry response from Chinese internet users.

China is in the middle of yet another crackdown on what it terms "online rumours", as the government tries to rein in social media, increasingly used by Chinese people to discuss politics, despite stringent censorship.

According to a judicial interpretation issued by China's top court and prosecutor, people will be charged with defamation if online rumours they create are visited by 5,000 internet users or reposted more than 500 times.

That could lead to three years in jail, state media reported, citing the judicial document. That is the standard sentence for defamation.

"People have been hurt and reaction in society has been strong, demanding with one voice serious punishment by the law for criminal activities like using the internet to spread rumours and defame people," said the court spokesman Sun Jungong.

"No country would consider the slander of other people as 'freedom of speech'," Sun said at a news conference, carried live by the People's Daily website.

The interpretation also sets out what is considered a "serious case" of spreading false information or rumours online, including those that cause mental anguish to their subjects.

Other serious cases involve the spreading of false information that causes protests, ethnic or religious unrest or has a "bad international effect".

Analysts say the new law could send a chill through China's online communities, which have been used with increasing frequency as a platform for reporting official malfeasance.

"This gives every corrupt local official a convenient tool to arrest anyone who criticises him," said Michael Anti, a prominent blogger and media commentator in Beijing. "It means the end of the online anti-corruption movement."

Unlike a similar internet crackdown in 2011 that mainly targeted dissidents and activists, the new law casts a wider net. Many writers, celebrities and internet entrepreneurs have millions of Weibo followers, and many will have to exercise greater caution in their posts, Anti said.

"This kind of political campaign, the anti-rumour campaign, can really create a chilling effect on the blogosphere," he said.

Users of China's popular Twitter-like Sina Weibo microblogging site expressed anger about the new rules.

"It's far too easy for something to be reposted 500 times or get 5,000 views. Who is going to dare say anything now?" wrote one Weibo user.

"This interpretation is against the constitution and is robbing people of their freedom of speech," wrote another.

State media have reported dozens of detentions in recent weeks as the government pushes a crackdown on the spreading of rumours.

The campaign comes as President Xi Jinping's newly installed government steps up its harassment of dissidents, showing no sign of wanting to loosen the party's grip on power.

China says it has a genuine need to stop the spread of irresponsible rumours, pointing to some of what authorities say are patently ridiculous things said online of late, including a story that a soup made from dead babies had gone on sale in Guangdong province.