Surfing the net at Sina's stall at Beijing computer show

But Sohu says the accusations are groundless and is threatening legal action against Sina for damaging its reputation.

Sina accuses Sohu of copying and plagiarising its content.

Sina says it filed the lawsuit after Sohu copied its mobile phone ring tones and image logos.

Claim and counter-claim

"Sina has endured large-scale plagiarism and rights violations of our product resources, including short messages, finance and sports channels," the company said in a statement on its website.

Sina is asking for more than $35,000 in compensation and an apology.

Sohu, however, dismisses the accusations as "ridiculous".

'Ulterior motives'

It says its rival has started a public relations battle in an attempt to raise its profile.

"Sina started a PR battle in China aiming to damage Sohu's reputation," it said in a statement.

"We reserve the right to take legal action against Sina for damaging Sohu's reputation," the statement said.

Sina and Sohu are listed on the high-tech Nasdaq exchange in New York, along with a third Chinese internet company, Netease.com.

They are known as China's "Big Three" dot coms.