BEIJING, March 29 (Reuters) - Chinese security forces sealed off parts of Lhasa on Saturday and Tibet's government-in-exile said it was investigating reports of fresh protests, weeks after the city was shaken by an anti-government riot.

The reports coincided with a visit by a group of diplomats, who were led on a closely guarded tour of the city that has been at the heart of unrest throughout China's ethnic Tibetan regions just months before the opening of the Beijing Olympics.

"We don't know how many people, but it seems it's quite a lot of people," Tenzin Taklha, a spokesman for the Dalai Lama said of the events in Lhasa. "I think it's timed with the visit of the diplomats."

The London-based International Campaign for Tibet said it had heard from three sources that security forces had surrounded Lhasa's main temples, Jokhang and Ramoche.

"The whole area has been shut down," the group's spokeswoman, Kate Saunders, said by telephone.

The trouble in the remote, mountain region that China's Communist troops entered in 1950 began with a series of peaceful, monk-led protests that culminated in a riot in Lhasa on March 14. Protests have since hit other Tibetan areas of China.

(Reporting by Lindsay Beck, editing by Myra MacDonald)

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