The railway snakes for 1,000km across 'the roof of the world'
See the railway
The Qinghai-Tibet line boasts high-tech engineering to stabilise tracks over permafrost and sealed cabins to protect passengers from the high altitude.
China says the 1,140km (710-mile) line will bring major opportunities to a poor region.
But critics fear it will be used by China to assert its control over a contested border region.
They also say the railway line threatens not only the delicate Himalayan environment, but also the ancient Tibetan culture.
Red ribbon
Mr Hu cut a red ribbon before the first train left Golmud in China's Qinghai province, carrying 900 passengers to the Tibetan capital, Lhasa.
"This is a magnificent feat by the Chinese people, and also a miracle in world railway history," Mr Hu said.
He said it showed China's people were "ambitious, self-confident and capable of standing among the world's advanced nations".
There have been protests against the railway, such as this in India
Musicians in traditional Tibetan and Chinese dress banged drums and cymbals as thousands of workers who helped to build the line looked on.
Minutes later, state TV showed another train departing Lhasa for Golmud.
On Friday, three foreign activists were briefly detained at Beijing's central railway station on Friday after unfurling a banner that read: "China's Tibet Railway: Designed to Destroy."
At its highest point, the railway will reach 5,072m (16,640ft) - beating by 225m a route through the Peruvian Andes that was previously the world's highest railway, the China Daily newspaper reports.
In parts, the train line has been built on bridges elevated above the most unstable permafrost.
Elsewhere, cooling pipes have been sunk into the ground to ensure it remains frozen to stabilise the tracks.
QINGHAI-TIBET RAILWAY Connects Lhasa to existing China rail network New 1,140km stretch cost $4.2bn World's highest railway, reaching 5,072m Oxygen to be pumped into each carriage Restaurant car's rice cooked in pressure cookers, to mitigate effects of high altitude Beijing to Lhasa to take 48 hours, cost $50-$160 one way
In pictures: New railway Railway raises fears Send us your comments
The train carriages have windows with ultra-violet filters to keep out the sun's glare, as well as carefully regulated oxygen levels with spare supplies to combat the thin air.
Zhu Zhensheng of the Chinese railway ministry called the new line a "major achievement" that will "hugely boost local development and benefit the local people".
But exiled Tibetan Lhadon Tethong said the railway was "engineered to destroy the very fabric of Tibetan identity".
"China plans to use the railway to transport Chinese migrants directly into the heart of Tibet in order to overwhelm the Tibetan population and tighten its stranglehold over our people," he said on a Free Tibet Campaign statement.
The Dalai Lama, Tibet's spiritual leader in exile since 1959, was more circumspect.
"The railway line itself is not a cause of concern for the Tibetan people," his spokesman, Thupten Samphel, said. "How it will be used is the main concern."
The railway goes from Golmud to Lhasa 
The first trains were flagged onto the Qinghai-Tibet Railway line, the world's highest railway line connecting the Tibetan capital Lhasa with Golmud in Qinghai, China. At its highest point in the Tanggula Pass, the railway line reaches an altitude of 5072 metres.

The line is designed for travel at speeds of 100 km/h on frozen ground to 120 km/h on non-frozen ground. The cabins are supplied with oxygen because of the thin air at the high altitudes. The windows have ultra-violet filters to keep out the sun's glare. Cooling pipes have been used to ensure the ground remains frozen.

Air trips to Tibet are expensive and road trips slow and dangerous.
Three trains, each carrying 900 passengers are scheduled to run daily along the line. The basic tickets cost 389 yuan to go from Beijing to Lhasa. The trip should take just under 48 hours. According to state television the tickets for the inaugural trip sold out within 10 minutes.

Speaking at the flagging-off ceremony in front of the Golmud railway station, the Chinese President Hu Jintao said, "The project is not only a magnificent feat in China's history of railway construction, but is also a great miracle of the world's railroad history,"

The Chinese government states that it spent more than US$95 million reducing the environmental damage of this railway. Passengers will not be able to open the windows of the train to litter. There will be a rubbish train to pick up refuse from various sites along the line.

Critics claim that the railway will be used to assert control over Tibet as the line can quickly transport Chinese troops to Tibet during unrest.

They also claim that the railway will damage both the environment and the culture of Tibet. Tibet is home to many endangered species such as the Tibetan antelope. 

On Friday, 3 women from the United States, Canada, and Britain were detained for displaying a banner saying "China's Tibet Railway, Designed to Destroy" at Beijing's main station.

"It's the first direct, low-cost, quick, and easy way for migrants to arrive in central Tibet. This will lead to the overwhelming of Tibetans and competition for scarce jobs. The eventual idea is the dilution of Tibetan language, culture and identity," said Lhadon Tethong, executive director of the New York-based Students for a Free Tibet.

"The railway line itself is not a cause of concern for the Tibetan people," said Thupten Samphel, spokesman for the Dalai Lama's government in exile. "How it will be used is the main concern."

"Despite the rosy economic forecasts and dire predictions of Chinese control, the train is unlikely to touch off social disaster or commercial windfall," said Andrew Fischer, an economics researcher at the London School of Economics.

"When the railroad opens the commercial service and more people move in and out of the region, I think it will be good for the outside world to know more about Tibetan culture," Zhu, the railway official, said. "No culture can develop and thrive in a closed environment."