Police have detained 28 ticket scalpers at the 1,300-year-old Potala Palace in Lhasa, Tibet Autonomous Region, accusing them of reselling tickets to tourists at high prices.

May to October is the busy season for visitors to the World Heritage site, as well as scalpers. The scalpers stockpile tickets and resell them at high margins, according to the public security bureau of Lhasa on Wednesday.

One ticket for Potala Palace could be sold for up to 1,000 yuan (150 U.S. dollars) which can be purchased for only 200 yuan through official channels.

To minimize damage to the wood and earth structure, the number of visitors is limited to 5,000 a day in peak travel season, failing to meet demand from the growing number of tourists. Visitors must book in advance through the palace's official website or at the ticket window with a valid ID card.

The scalpers used their own ID cards or borrowed from others to secure the reservation in advance, then resold tickets to those who failed to make reservation. Some of them even lied to tourists at the site that there were no more tickets, forcing them to participate in group tours that would give the scalpers kickbacks.

Police said that the ticket scalpers "have severely disturbed the market order."

The Potala Palace was built by Tibetan King Songtsa Gambo in the 7th century and expanded in the 17th century. It was listed as a UNESCO World Heritage Site in 1994.

Around one million visits were made to the palace last year, according to the site's administrative office.