By Michael Bristow
BBC News, Beijing
On 10 March 1959 Tibetans rose up in rebellion against the Chinese government. That rebellion failed and the Dalai Lama fled.
There is said to be a heavy security presence on Lhasa's streets
According to exiled Tibetans, it was a justified uprising aimed at freeing the Himalayan region from the clutches of the oppressive Chinese.
Beijing sees it differently. It says it was a plot by Tibet's upper-classes, who wanted to maintain their feudal hold over their enslaved people.
Seldom has there been two such different versions of the same historical event.
And this difference of opinion continues today. Exiled Tibetans and the Chinese government are far apart on what is happening now in the region.
The 50th anniversary of the uprising comes at a sensitive time for Tibet, a year after riots and protests broke out in Tibetan communities across western China.
China says the region has returned to normal, but people who have visited Tibetan areas say there is heavy security and the situation is tense.
Tibetans and Chinese have long differed over the status of the region, which Beijing says has been a part of China since the 13th century.
Some Tibetans argue that for much of that time Tibet was able to administer its own affairs, a situation that only ended with the victory of the communists in China in 1949.
China says Tibet is part of its territory
Tibet enjoyed long periods of autonomy before the 20th Century
1950: China launched a military assault
Opposition to Chinese rule led to a bloody uprising which began on 10 March 1959
Tibet's spiritual leader, the Dalai Lama, fled days later and crossed into India on 31 March 1959
China's new rulers soon established their control over the far-flung region. Beijing still refers to this as a "peaceful liberation"; exiled Tibetans say it was an invasion.
The Central Tibetan Administration, Tibet's government-in-exile based in India, says during this period the Chinese oppressed ordinary Tibetans.
"When people are oppressed, they are likely to rise up against the oppressor," it says in an official statement about the uprising.
The rebellion appears to have begun in the mid-1950s and rumbled on until it engulfed the whole region in March 1959.
"With the support of foreign anti-China forces, the reactionary clique of Tibet's upper class elaborately plotted and instigated a full-scale armed rebellion," reads a recently published Chinese white paper on Tibet.
But fearing for his life, the Dalai Lama, who was then Tibet's political and spiritual head, fled his homeland soon afterwards. A total of 80,000 Tibetans followed.
"He decided to escape from Tibet so that he could continue to serve the Tibetan people from a free country," says the Tibetan government-in-exile.
After two weeks trekking across high mountains, the Dalai Lama made it to India, from where he has been based ever since.
According to exiled Tibetans, others have not been so lucky.
"Over 1.2 million Tibetans, one fifth of a population of six million, have died as the direct result of China's invasion of Tibet," says the government-in-exile.
Serfs and slaves
Over the last half a century, the stated aim of this exiled government has been to bring greater freedom to Tibet.
But this is not how China sees it. It says the uprising in 1959 was an attempt by Tibet's traditional religious leaders to maintain serfdom.
Tourists and journalists have been banned from Lhasa in recent weeks
After the uprising, China says it set up a democratic government in Tibet.
"The Central People's Government... led the Tibetan people to... realise the life-long wish of a million serfs and slaves of being their own masters," says China's white paper on Tibet.
China believes it was no less significant that the emancipation of slaves in the United States.
To commemorate the occasion, the Chinese government has just announced that 28 March will now be known as Serfs' Emancipation Day.
Many Tibetans living in the region will no doubt prefer to remember 10 March and the failed uprising.
'No riots expected'
It was on that day last year when a series of protests began against Chinese rule in Lhasa. These quickly spread to other Tibetan areas.
The government-in-exile says China's "ruthless" clampdown following the protests left 219 peaceful protesters dead.
Last year, the 10 March anniversary triggered bloody riots a few days later
It says about 5,600 people are under arrest or in detention, with 1,000 missing.
China says 18 civilians and policemen died in what it terms a riot. It says everyone who has not been prosecuted has now been released.
To prevent a repeat of those protests this year, there is a heavy security presence in Tibetan areas, including those outside the Tibetan Autonomous Region, the official name for the main Tibetan area.
Security seems to be tightest in Lhasa, the Tibetan capital, where personnel wearing military uniforms patrolled the city streets, according to one recent visitor.
Foreign tourists and journalists are banned from visiting.
There was also a heavy security presence in Tibetan areas in western Sichuan Province, according to another eyewitness.
But some other areas appeared more relaxed. A BBC journalist recently managed to visit the village where the Dalai Lama was born in Qinghai Province.
Tibetan Qiangba Puncog, head of the government in the autonomous region, said he expected no major riots this year.
But these officials know that 10 March is an important date for Tibetans, and the heavy security presence suggests China is not leaving that to luck.