Display screens are shown at the gaming table inside a casino during the opening day of Sheraton Macao Hotel at the Sands Cotai Central in Macao, Sept 20, 2012. [Photo/IC]

China's biggest online search engine, Baidu, said on Monday that it had notified police of illegally altered links that redirect users to gambling sites. The links had found their way surreptitiously into Baidu's search results at night.

For the past two months, a search after 10 pm for keyword "Grand Lisboa", a well-known casino in Macao, delivers top results linked to online gambling sites marked as commercial promotions. The search results will return to legitimate information about the casino after 9 am, Beijing News reported on Monday.

Baidu responded to the report with its own investigation.

"Baidu focuses on the credentials of the company registered for promotions. But the contents of promotional websites cannot be monitored in advance. So we make sure every website is screened as soon as they go live," said Gu Guodong, executive director of marketing at Baidu, during a press briefing in Beijing on Monday.

On average Baidu screens 470 million pieces of content every day, or 5,000 per second. Computers can eliminate 95 percent of the illegal information, but the rest must go through manual screening. The whole process can take up to four hours.

The Nasdaq-listed company admitted that it has to do more to improve the efficiency of such screening to prevent people from posting illegal promotional content.

Gambling is illegal on the Chinese mainland. Baidu blocked 8,623 gambling sites and 7,239 gambling-related keywords in the first three months of the year, the company said.

To bypass China's internet administration, illegal gambling sites steal the details of a legitimate company to use as registration information on Baidu. For example, eight gambling sites were registered in the name of a car parts dealer in Central China's Henan province, Beijing News reported.

A staff member at that company told the newspaper that they had no idea the company's registration information had been stolen and was being used for any sort of online promotion.

Baidu said it monitored the illegal activities from car parts company's stolen Baidu account. The gambling promotion appeared at 11 pm on June 25 and was removed by Baidu at 6 am the next day.