Chinese President Hu Jintao cut a giant red ribbon at a nationally televised ceremony in the western city of Golmud as the first train left for the Tibetan capital of Lhasa carrying 600 passengers. Musicians in Chinese and Tibetan costumes banged on drums and cymbals.
Minutes later, state television showed a second train pulling out of Lhasa traveling toward Golmud. A third train was due to leave Beijing for the Tibetan capital later in the day.
"This is a magnificent feat by the Chinese people, and also a miracle in world railway history," Hu said. He said it showed China's people were "ambitious, self-confident and capable of standing among the world's advanced nations."
The 710-mile rail line crosses mountain passes up to 16,500 feet high and large stretches of ground that is frozen year-round. Specially designed train cars have oxygen supplies to help passengers cope with the thin air and window filters to protect them from ultraviolet rays, while high-tech cooling systems keep the railbed frozen and stable.
The $4.2 billion train project is part of the Chinese government's efforts to develop poor, restive areas in China's west and bind them more closely to the country's booming east.
Its opening coincided with a major political anniversary, the 85th anniversary of the founding of the ruling Communist Party.
But activists complain the railway will bring an influx of Chinese migrants, damaging Tibet's fragile ecology and diluting its unique Buddhist society. They say most of its economic benefits will go to migrants from the east.
On Friday, three women from the United States, Canada and Britain were detained after unfurling a banner at Beijing's main train station reading, "China's Tibet Railway, Designed to Destroy."
Protests were planned Saturday outside Chinese embassies around the world.
Chinese officials acknowledge that few Tibetans are employed by the railway but say that number should increase. The government also says it is taking precautions to protect the environment.
Xinhua lashed out at critics on Saturday, calling them hypocrites who want Tibet to remain undeveloped and a "stereotyped cultural specimen for them to enjoy."
"Why shouldn't Tibet progress like the rest of the world?" the commentary said.
The railway, sometimes referred to as the "Sky Train" in Chinese, is projected to help double tourism revenues in Tibet by 2010 and reduce transport costs for goods by 75 percent, the official Xinhua News Agency said. Until now, goods going to and from Tibet have been trucked over mountain highways that are often blocked by landslides or snow, making trade prohibitively expensive.
Communist troops marched into Tibet in 1950 and Beijing says the region has been a Chinese territory for centuries. But Tibet was effectively independent for much of that time.
Chinese officials have wanted to build a railway to Tibet for decades but were put off by the engineering challenges.
The project was launched in earnest in 2001 after engineers decided they could deal with the high altitude and temperature extremes of the Tibetan plateau. In some places, crews building the line worked at such high altitudes that they were forced to breathe bottled oxygen.
The railway's highest station will be in Nagqu, a town at 14,850 in the rolling grasslands of the Tibetan plateau.
According to Xinhua, the highest point on the line is the Tanggula Pass at 16,737 feet, which the government says is a world record. Peru's Lima-Huancayo line claimed the highest record previously, rising to above 15,748 feet.