BEIJING, Oct 1, 2007 (AFP) - Millions of Chinese began a week-long National Day holiday Monday, packing rail stations and airports as tens of thousands flocked to Tiananmen Square and other major tourist sites.

Up to 150 million people are expected to travel this week -- according to police estimates -- boosting consumption and filling hotels and restaurants as the nation celebrates the 58th anniversary of the People's Republic of China.

More than 100,000 people gathered on Tiananmen Square in central Beijing early Monday morning for the flag-raising ceremony, Xinhua news agency said, while at least that many remained on the square throughout the day.

The police presence underneath the portrait of founding father Mao Zedong on the Tiananmen Gate was heavy, with state press adding that up to 1,200 plain-clothes policemen would be on duty in the heart of the capital during the holiday.

"China's Ministry of Public Security has warned all local public security departments to stand alert during the week-long National Day holiday to ensure social stability and people's safety," Xinhua said.

Commercial areas in Beijing's city centre were also packed with shoppers, although traffic on the capital's streets was far from its normal gridlock, with residents appearing to prefer mass transit to private cars.

Beijing was expecting about 1.7 million people to visit the capital and an equal number to leave during the "Golden Week" holiday, reports said.

Up to 4.2 million tourists were expected to visit Shanghai, while tourist sites around the nation's booming eastern metropolis were also expected to be swamped with people from the city, they added.

According to the China News Service, airline seats are heavily booked for the beginning and end of the holiday, with planes on major routes nearly 90 percent full.

Tickets on major railway lines were also hard to get as travellers packed trunk lines to key cities and destinations such as Beijing, Shanghai, Guangzhou, Harbin, Dalian and Xiamen, it said.

Hotels at major tourist sites such as the Buddhist grottoes at Dunhuang, and the scenic retreats of Mount Emei, Mount Tai and Yulong mountain were up to 70 percent booked, it added.