An expatriate university teacher surprised locals by impersonating an ancient Chinese emperor on the busy Metro Line 1 on Monday morning in a Halloween stunt.

A picture posted on a microblog shows William Lefeuvre, a Canadian teacher at the University of Shanghai for Science and Technology, in period costume on the Metro.

A passenger photographed the man in a golden dragon robe and put the picture online. It was quickly forwarded thousands of times and online users have been commenting on and guessing his identity and purpose.

"It is so funny to see a foreign man dressed as the emperor," said an online user identifying himself as "a clever little boy."

Another said: "We are equal in the 21st century because the emperor also needs to take the crowded subway and no one gives him a seat."

The emperor's identity became clear yesterday after his students recognized him. William Lefeuvre, a Canadian, has taught project management at the Sino-British College of the University of Shanghai for Science and Technology for two years.

Students call him Bill. A class of 80-plus students exposed him after seeing him in the emperor's clothes.

"Many Chinese do not know much about Halloween," he said. "It is part of my culture and history and I wanted to share it with my students."

Other expat teachers at the college donned Halloween costumes depicting witches and other characters.

Lefeuvre said he had walked through the city in costumes depicting KFC Man and Santa Claus several times in the past two years without creating a stir.

Metro passengers smiled or bowed to him in the dragon robe. One even said: "Long live the emperor."

He said: "I took public transport because I enjoyed seeing the smiles on people's faces."

The teacher said he also wanted to encourage students to overcome their shyness, adding: "It is a good opportunity to demonstrate how to launch a successful event that creates laughter and fun."