Do not give away your used Expo tickets to strangers, police officers warned visitors.

Like many others who had a long, tough day in the Expo Garden, Ms Yu, a 25-year-old reporter, left the site from Gate 6 as night fell. When she departed, she was approached by a group of strangers.

"Whispering, they asked if I had a used ticket to sell for 5 yuan (73 cents)," said Yu. "They already had several in hand."

"There were about 7 or 8 of them, men and women, who repeatedly asked the same question to visitors leaving the Expo," Yu said.

The following day she crossed paths with another group, who were willing to pay 5 to 10 yuan for a used ticket and occasionally offered an Expo trinket in exchange.

Like many other visitors, she tried to avoid these scalpers, commonly known as huangniu in China, who she had previously encountered at railway stations.

While railway station scalpers are able to resell tickets to travelers, she could not imagine what further use might be made of Expo tickets, especially so late in the day.

"When a ticket passes the checking machine, it is marked with an indentation of '2010 EXPO'," said a police officer, surnamed Huang, who is stationed at the Yaohua Road entrance to the garden.

"If it's checked manually, a punching machine will leave a hole in the ticket. The difference separates a used ticket from a new one.

"We have repeatedly told visitors to only purchase tickets from official merchandisers," Huang stressed. "And now we are reminding them not to sell their used tickets to strangers."