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.