Shanghai police have taken over a case in which at least four foreigners were seen in a subway carriage eating a meal and drinking from goblets while sitting at a folding table on Friday, the subway operator said.

"Although an eating ban was not written into the city's regulations for the Shanghai Metro system, the police will examine whether the behavior disturbed public order and the normal operation of the subway, and then decide if the foreigners will receive any punishment," said Yin Wei, media communications manager of Shanghai Shentong Metro Group.

Regulations came into effect in 2014 listing 11 forbidden behaviors, including interfering with a train and entering forbidden areas such as tunnels without permission - acts that can result in detention - and smoking or selling products on trains or platforms, which can result in fines of between 50 yuan and 500 yuan ($7 and $72). Other behaviors that disturb public safety and order may also be punished.

Police authorities declined to comment on Tuesday.

The incident, which has become an online sensation, was first exposed by a Sina Weibo user in a post on Friday.

The original post was deleted on Tuesday, but it had already been forwarded by individuals and media outlets. Many internet users have said the behavior was ridiculous and should be punished to prevent copycats.

"Foreigners shouldn't be treated in a more friendly way under our laws and regulations. Suppose the offenders were locals: I don't think they would have had the freedom to do such an unbelievable and unbearable thing at will in public," one netizen commented.

In 2014, Shanghai Metro encouraged people to ask others to refrain from eating on trains to avoid annoying fellow travelers.

"When finding someone eating on the subway, passengers can call our service hotline, alert the staff on the platforms or even call the police," Yin said.

In May, two men who drank while eating peanuts and eggs at a folding table they had brought into a subway carriage in Guangzhou, Guangdong province, were placed in detention for five days for disturbing public order.

The local police said their behavior violated municipal regulations.

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