LHASA, CHINA, OCT. 5 -- Chinese authorities flew two planeloads of security forces into this Tibetan capital today and ordered "separatist elements" to surrender by Oct. 15 or face severe punishment.

In the wake of violent anti-Chinese rioting last week, the heightened security measures appear aimed at preventing further proindependence demonstrations here with the approach of Wednesday's 37th anniversary of the Chinese Army's invasion of Tibet.

Despite the government warnings, monks have told foreign reporters that they are prepared to fight the Chinese with their bare fists and will use guns if they can get them. The monks say they have been encouraged by certain physical signs and portents they say foreshadow a new era for Tibet and the return of the Dalai Lama, the exiled Tibetan Buddhist leader.

Underground leaflets and small posters have appeared in recent days calling for united action against the Chinese. The posters are put up at night and torn down by police the next day.

"Ten people have been killed," said one small poster found in an alley in the center of the city. "Now it is time to act. Ten, 20, or 30 more may die. . . . We must all make sacrifices."

The fervor of the monks has been evident everywhere in the center of the city in the past few days. Within sight of a police station burned in a demonstration last Thursday that resulted in at least six deaths, Tibetan pilgrims have been prostrating themselves at the Jokhang Temple, Tibet's most sacred shrine. But there were reports today that police had occupied part of the temple, the site of a separatist demonstration Sept. 27.

{A statement from Buddhist monks at three monasteries distributed to tourists in Lhasa called on the United Nations to support their cause, The Associated Press reported from Beijing.

{"The Chinese have ruled violently in our country and we want them to leave Tibet," the statement said. "Tibet belongs to the Tibetans and the Tibetans have the right to have autonomy."}

China forcibly annexed the strategic region in central Asia, often referred to as the "rooftop of the world," three decades ago, but relations between the 1.7 million Tibetans and the estimated 400,000 Chinese civilians and troops stationed here are strained.

Chinese helicopters have appeared overhead in recent days, and loudspeakers blare warnings against rebellious activities.

A 10 p.m. curfew is still in effect, police have set up roadblocks around the city, and three Buddhist monasteries, whose monks organized two of the demonstrations in the past eight days, remain sealed off.

One of the monasteries, the Sera, on the northern outskirts, was noted in the past for its warrior monks and Sera monks led last Thursday's demonstration.

Meanwhile, officials of the Chinese airlines office in the city of Chengdu, Sichuan Province, the main transfer point for flights to Lhasa, said that beginning Tuesday, no new tickets would be issued to foreigners wishing to visit Lhasa for the next 10 days. No reason was given for this move.

Police increased security measures at the Lhasa airport today and appeared to be attempting to apprehend any Tibetans entering or leaving who have links with the Dalai Lama, who lives in exile in India.

The Chinese have blamed him for the demonstrations, the first of which, on Sept. 27, coincided with his visit to the United States. "The bloodshed in Lhasa has proved how much calls for Tibetan independence go against the interests of the Tibetan people," the official English-language China Daily newspaper said today in an editorial.

{In New Delhi, the Dalai Lama's office denied he was responsible for the violence, Reuter reported. "Chinese authorities are responsible for what is happening in Tibet," spokesman Tashi Wangdi said.}

Tibetan sources said 10 or more Tibetans died and more than 40 monks have been arrested. During last Thursday's protest, witnesses said, the police, after at first holding back, lost their nerve in the face of the stone-throwing mob and fired repeatedly on unarmed demonstrators.

Although the government is offering lenient treatment to those who surrender by Oct. 15, scores of monks were reported to be missing from their monasteries and in hiding. Foreign doctors visiting Tibet as tourists treated a number of the monks and others who were wounded in the rioting. Most of the injured suffered from gunshot wounds and a few from severe burns, they said.

The wounded monks are avoiding Chinese hospitals because they are afraid they will be arrested if they seek medical treatment.

Authorities have put up posters warning foreigners not to participate in demonstrations or photograph them. Several foreigners were seen throwing stones alongside the demonstrators last Thursday, according to foreign travelers. A police bullet passed through the makeshift turban worn by one young backpacker, identified as an American, according to foreign travelers.

Two Americans visiting Lhasa were detained for questioning by police because they carried on their bags Tibetan nationalist flags showing the snowy mountains and the lion that has religious significance for Tibetans. The same symbol was part of a flag carried by demonstrators during the Sept. 27 protest.

The two men, Dr. Blake Kerr, 29, and John Ackerly, 30, a lawyer from Washington, said they were accused by police of "endangering national security" and were questioned three to four hours daily, for three days. They were allowed to return to their hotel each night.

Foreign reporters working in the Tibetan capital in recent days have rarely been able to place telephone calls overseas or to other cities in China or to file their reports by telex. Calls from the outside have also been cut off. {But some western correspondents were allowed to use public telex facilities today for the first time, United Press International reported from Lhasa.}