UPDATE 1-Heavy security as Tibetans mark Dalai Lama's exile

(For more stories on Tibet, click on [nPEK257634]) (Adds roadblock, paragraphs 7-8; Tibet governor, paragraphs 16-17)

By Lucy Hornby

TA'ER, China, March 10 (Reuters) - China tightened security across ethnic Tibetan areas on Tuesday, aiming to head off potential unrest on the sensitive 50th anniversary of a failed uprising that prompted the Dalai Lama's flight into exile.

The Nobel Prize-winning Buddhist monk, whom China brands a separatist, marked March 10 with a speech in India calling for "meaningful autonomy" for his homeland, and slammed Beijing for bringing "untold suffering and destruction" to Tibet.

Monks, who have initiated many Tibetan protests in recent years, told Reuters they were under close surveillance and riot police blocked roads and turned away foreign journalists from parts of Sichuan, Gansu and Qinghai provinces.

Tibet proper has always been off-limits to all foreigners without zealously controlled permits.

"It is doubly tense today and this morning the security forces made a show of marching around," said one young devotee at the Kumbum monastery, the legendary birthplace of the lama who founded the Dalai Lama's sect of Buddhism.

"Things seem quiet here but there are cameras throughout the monastery and many of the tourists are actually plainclothes security," he added, asking not to be named because the tense situation made talking to journalists risky.

Armed police turned back Reuters reporters some 50 km (30 miles) from the heavily Tibetan Qinghai town of Tongren.