Two of China's leading Internet firms have been forced by the government to issue public apologies over a nasty spat marked by accusations of unfair market practices and privacy infringement.

Tencent, parent company of the popular instant messaging service QQ, and security software developer Qihoo 360 issued the apologies late Sunday after being ordered to do so by the Ministry of Industry and Information Technology.

The public feud began in September when Tencent encouraged users to download its upgraded security software. Qihoo 360 subsequently accused QQ of trying to scan its users' personal data and it issued tools to block QQ components.

Tencent earlier this month announced that QQ would no longer function on computers using Qihoo 360 products, while Qihoo 360 in turn took similar moves.

The row triggered harsh criticism from web users in China, which has the world's largest online population, with at least 420 million people using the Internet.

In a statement on its website, Qihoo 360 said "we hearby apologise to society and netizens and will stop the mutual attacks between the companies."

The ministry had earlier on Sunday told the companies to "publicly apologise to society," saying in a statement the feud had "sparked user discontent and caused vile social influences."

"We hereby sincerely apologise to all the users that have been troubled by this incident," Tencent said in its statement.

Tencent claims QQ has more than 600 million active user accounts, while Qihoo 360 boasts more than 300 million, Chinese media have reported.

The ministry statement had said the two firms must "stop attacking each other" and ensure that their software is compatible.

Authorities will investigate whether either company violated any laws amid the dispute, it added.

"The two companies should learn lessons from this incident... strengthen professional ethics, strictly regulate their practices and make sure similar incidents never happen again," the statement said.