Chinese authorities have asked managers of the country's popular tourism sites to take safety precautions during the current weeklong National Day holiday period, after thousands of tourists were stranded for hours in a national park.

The China National Tourism Administration issued a notice to the managers on Thursday, one day after more than 4,000 tourists were stranded in the Jiuzhai Valley National Park in the southwest province of Sichuan following delays in shuttle bus services. The delays paralyzed the area's transportation system, and military police were summoned to keep order.

The weeklong holiday, known as Golden Week, is a peak travel period in China, with hundreds of millions of Chinese taking advantage of the extended break. The tourism administration said more than 6 million people had visited 125 sites across China in the first three days of the Oct. 1-7 holiday period, up by 8.4 percent from last year.

Photos in state media have shown massive crowds in popular tourist sites across China, raising safety concerns.

The People's Daily said the historic Mogao Caves in western China had a record number of more than 20,000 visitors on Thursday, raising the carbon dioxide inside some caves to dangerous levels that could ruin the ancient murals.

In the scenic Jiuzhai Valley park, known for its colorful lakes and vibrant fall foliage, the obstruction of shuttle buses by tourists triggered a chain reaction that halted the area's transportation operations, the park management said in an open apology Thursday. The last tourists did not get to leave the site until 10 p.m., and angry visitors swarmed the ticket office, demanding refunds.

In the eastern city of Hangzhou, more than 1 million people flooded to scenic West Lake on Wednesday, setting a single-day record. Other Chinese cities also have reported record numbers of tourists. Despite complaints of long lines, no other major incident has been reported.

In its notice, the national tourism administration asked managers to beef up management, release tourist information in a timely manner, provide additional services and solve problems promptly to prevent incidents.