BEIJING, March 14 (Reuters) - Chinese authorities sealed off three monasteries in Tibet, reports said on Friday, after a wave of rare street protests in the remote, Buddhist region whose rule has become a focus for critics ahead of the Beijing Olympics.

The demonstrations, which also spilled into Chinese provinces populated by Tibetans, began earlier this week after marches around the world to mark the 49th anniversary of a failed uprising against Communist rule.

"All three monasteries are closed off to tourists," the Washington-based International Campaign for Tibet said in a statement, citing tourism operators. "There is an intensified atmosphere of fear and tension in Tibet's capital."

On Monday, 500 monks from the Drepung monastery staged a march in Lhasa, which was later followed by action from monks at the Lhasa-area Sera and Ganden monasteries. Security personnel fired tear gas on at least one of the demonstrations, reports said.

Tibet's spiritual leader, the Dalai Lama, fled into exile in India after the uprising in 1959, nine years after Chinese troops invaded.

This week's shows of defiance are likely to worry China's leadership as it seeks to secure a stable environment for the Games, which open on Aug. 8.

The U.S.-government funded Radio Free Asia (RFA) reported that monks from Sera were on a hunger strike, demanding the withdrawal of Chinese paramilitary force from the monastery compound and the release of monks detained earlier this week.

Two monks from Drepung were in critical condition after attempting suicide by slitting their wrists, RFA said.

The number of Tibetans detained could not be confirmed, but the watchdog groups said they expected government reprisals.

"There are indications that the authorities have begun a process of investigation in monasteries that could lead to detention and torture," the International Campaign for Tibet said.

The New York-based Human Rights Watch called on China, as well as Nepal and India, which have seen similar demonstrations, to release detained Tibetans.

"Peaceful demonstrations are protected under international and domestic laws and they should be permitted, not violently dispersed," Sophie Richardson, the group's Asia advocacy director, said in a statement. (Reporting by Lindsay Beck; Editing by David Fox)

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