By Jane Macartney

BEIJING, Sept 4 (Reuter) - Independence activists in the rebellious region of Tibet twice bombed a Chinese memorial plaque in Lhasa in the run-up to official celebrations to mark 30 years of Beijing rule, Chinese sources said on Monday.

The bombings took place in the two weeks before China staged a gala parade in the capital, Lhasa, last Friday to mark the 30th anniversary.

During the celebrations top officials sharply criticised the exiled Dalai Lama, accusing him of trying to split China.

Officials clamped a security net around Lhasa during the anniversary celebrations, the Chinese sources said.

Police carrying AK-47 semi-automatic rifles patrolled the streets to stop anti-Chinese demonstrations, they said.

Additional troops were deployed in the city on the night of August 31, the eve of the gala parade, because the number of police was insufficient to ensure security, they said.

Local officials were determined to avoid any trouble during a visit by Vice Premier and Politburo member Wu Bangguo as the chief guest for the 30th anniversary celebrations, they said.

But on August 27, independence activists infiltrated the security cordon and set off a small bomb at the plaque in central Lhasa, erected by former Communist Party chief Hu Yaobang, the sources said.

A similar explosive device went off two weeks earlier, also slightly damaging the plaque inscribed with the calligraphy of the late party chief to commemorate what China calls its liberation of Tibet.

People's Liberation Army troops entered Tibet in 1950 and Beijing solidified its rule over Tibet in 1965 by setting up the ostensibly autonomous government.

The stone tablet on a street in central Lhasa was repaired overnight following both incidents, said the sources familiar with Tibet.

However, officials have since cordoned off the area and forbidden photographs of the plaque, they said.

Since the late 1980s, Chinese security forces have suppressed numerous anti-Chinese uprisings in Lhasa and other Tibetan towns, usually led by lamas loyal to the Dalai Lama, who fled into exile in India after an abortive rebellion against Chinese rule in 1959.

On Friday, as soldiers, monks and brightly decorated floats paraded through Lhasa in the anniversary parade, nine Tibetan women exiles, their mouths gagged with scarves to represent repression in their Himalayan homeland, staged a daring protest at an international women's forum in Beijing.

"Our women in Tibet are denied the freedom of expression," said demonstrator Dorjil Carroll. "We are protesting against that (Chinese rule)."

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