Chinese troops rounded up suspected separatists and kept a tight lid on Tibet Friday as the embattled region marked the 30th anniversary of a failed attempt to cast off Chinese rule.

Police and the military already had detained many Tibetans in house-to-house searches and planned to move swiftly to crush any demonstrations to commemorate the anniversary, according to Tibetan and foreign sources.A Tibetan woman, contacted by telephone Friday, said the capital was calm and that the army occupied all roads.

"There are new arrests. . . . There are many, but we don't know how many," she said.

A man came on the line.

"Don't talk a long time," he said. "There are a lot of police. We can't talk long."

China imposed martial law in Lhasa Tuesday to quell three days of anti-Chinese riots that took 16 lives, according to official count. Western travelers say most Tibetans put the death toll at between 20 and 30. Some estimates ranged much higher.

All foreign tourists were ordered to leave Lhasa by Thursday. It was believed the last group departed Friday morning.

Another Tibetan woman reached by telephone Thursday evening from Beijing said Chinese troops began rounding up large numbers of Tibetans after most foreigners left.

China's national radio Friday said large groups of protesters turned themselves in and that police said those who gave themselves up would be treated with leniency.

It said some shops had reopened in the mountaintop city of 70,000, and the Barkhor, the central marketplace, was returning to life.

During the riots that began Sunday, mobs of Tibetans demanding independence from China and chanting loyalty to their exiled spiritual leader the Dalai Lama threw stones, ransacked Chinese-owned shops, set fire to Chinese property and attacked government office buildings.

The Dalai Lama spoke Friday to about 5,000 ethnic Tibetans in Dharmsala, India, who gathered to mark the 1959 uprising. He urged Tibetans to demonstrate against Chinese rule but avoid violence.

"The non-violent struggle is the best because of the recent events," he said. "Have faith in the struggle and continue demonstrations so that the world will know Tibet is suffering under Chinese rule."

He called on Chinese leader Deng Xiaoping to end violence in the Tibetan capital, according to a statement issued by his headquarters in New Delhi.

"I urge your personal intervention for bringing an immediate end to the repressive measures against innocent Tibetans and lifting of the martial law in Lhasa," the statement said.

Thousands of Tibtetans also demonstrated in New Delhi and the Indian city of Madras Friday.