BEIJING - Police have launched an investigation into a food poisoning case where 80 people, mostly tourists, fell ill after eating contaminated food at a restaurant in a Beijing suburb on Friday.

As of Sunday, more than 40 people were still hospitalized in stable condition and will hopefully recover within the week, Xinhua News Agency said on Sunday. The rest had already been discharged from the hospital.

The 80 food poisoning victims began to develop symptoms, including nausea and vomiting, after finishing a 16-course lunch at the restaurant, Shui'an Shanba, a holiday resort in the outer district of Huairou.

Preliminary tests showed that some of the sufferers had the high blood pressure drug clonidine in their blood, according to the hospital where they were treated.

Whether the contamination resulted from someone intentionally tampering with the food requires further investigation, according to a statement issued by the Huairou public security bureau over the weekend.

The police closed off the site for their investigation.

Some local newspapers reported that the clonidine was likely to have been deliberately added to the food, as the drug is not widely available in Beijing.

When injested, it can cause lightheadedness, dry mouth, dizziness or constipation, experts said. An overdose could lead to liver or kidney dysfunction or even a coma.

Most of the sick were treated at the 307 Hospital of the People's Liberation Army and the nine gravely ill patients were taken to the Chaoyang Hospital in northeast Beijing.

The nine, including a two-year-old boy and his parents, all had clonidine in their blood, a doctor in the hospital's emergency room said on condition of anonymity.

He assumed the victims' food was probably contaminated with the drug, as it was otherwise unlikely for dozens of diners to suffer from an overdose of the drug at the same time.

"Most of them responded little, even to my flashlight, when I checked their pupils on Friday," said Hao Fengtong, a physician with the department of vocational illness and toxicology.

Police ordered the resort hotel, which opened 10 years ago, to suspend operations until the case is cleared.

- China Daily/Asia News Network