LHASA, CHINA, OCT. 4 -- Chinese soldiers and police tightened their grip on the Tibetan capital of Lhasa today, blocking roads and sealing off monasteries after as many as 19 persons were reported to have been killed in a riot on Thursday against Chinese rule.

The official death toll has been put at six but reliable Tibetan sources said up to nine demonstrators and 10 policemen had died. The dead demonstrators included three Buddhist monks, they said.

All main roads out of Lhasa were blocked by police, who turned back reporters trying to visit outlying monasteries. One policeman pointed his submachine gun at a reporter and said there had been a disturbance at Ganden Monastery east of Lhasa. Sera Monastery was closed off and one policeman said it might remain sealed off for a week. The Potala Palace, former home of Tibet's spiritual leader, the Dalai Lama, was also sealed off.

Roads to Xhigatze in the west and Golmud in the north were blocked and western travelers said they and local people were forced to return to Lhasa.

A 10 p.m. curfew was in force and loudspeakers tonight broadcast official statements as police jeeps patrolled the empty streets.

{This report was telephoned from Lhasa in spite of orders given to hotel managers and telephone operators not to let foreigners communicate with the outside world, Reuter reported. Before it was completed, the operator announced that she was cutting the line, and did. The Associated Press sent out a dispatch with a western tourist leaving the city.}

Tibetan nationalists said the crackdown was aimed at preventing more demonstrations on Oct. 7, the 37th anniversary of the entry into Tibet of the Chinese Army.

In the first detailed account of Thursday's riot, the worst officially reported violence here for more than a decade, the official People's Daily today accused two foreigners of involvement. The newspaper said the foreigners, who were not identified, were among the crowd that burned a police station and attacked policemen with stones and bricks.

China's state television, in a rare broadcast of antigovernment violence, also showed film of the riot for the first time today. The broadcast showed crowds, including monks in robes, stoning police and setting fire to fire engines and other vehicles. It also showed the gutted police station.

Meanwhile, in Beijing, the U.S. Embassy said two Americans who had been detained for several hours in Lhasa last week were released and ordered to leave China by Oct. 10. An embassy spokeswoman could not say why they had been detained or whether they had taken part in Thursday's riot. But she said the two, who had Tibetan flags in their luggage, were still in Lhasa. Their passports, which had been confiscated several days ago, were returned to them.

{Dr. William Ackerly, father of one of the men, said State Department officials informed him today that his son, John Ackerly, 30, a Washington lawyer, and his son's friend, Dr. Blake Kerr, 29, had been released.

{"We're very relieved," said Ackerly, in a telephone interview from Cambridge, Mass. He said he did not know why they had been detained. The two men had been traveling as tourists and were on their way to Nepal and India, he said.}

Chinese officials said police did not fire at rioting crowds on Thursday, China's National Day, in spite of more than a score of foreign witnesses who gave contrary accounts. The official Chinese news agency has said six people were killed.

{Kerr said six Tibetans were killed Thursday, including a child of about 8 and a 16-year-old youth whose neck was shattered by a bullet, according to a dispatch by Agence France-Presse that was carried out of Lhasa today by a passenger flying to Beijing. Kerr said he and other foreigners treated 11 Tibetans, including women and children, for bullet wounds.}

The curfew was imposed last night. Army helicopters buzzed several times over the Tibetan capital and shortly before midnight, a convoy of about 30 police trucks and jeeps patrolled the streets.

Thursday's protest started as a peaceful march around Tibet's holiest shrine, the Jokhang Temple in the center of Lhasa, but turned into a battle between Tibetan nationalists and police after about 50 demonstrators were arrested. After a police station and a barricade of official vehicles were set ablaze, police shot into the air and then opened fire into the crowd, witnesses said.

The riots, which lasted more than five hours, were some of the bloodiest protests against Chinese rule since an abortive rebellion in 1959 after which the Dalai Lama fled into exile. Tibetan sources said demonstrations took place in other Tibetan towns on Thursday. Their reports could not be verified independently.

During Thursday's rioting, some monks fought hand-to-hand with plainclothes police on the roof of the Jokhang Temple.

About 1,000 monks live in Lhasa under quotas set by the Chinese authorities, a small fraction of the pre-1959 religious population. Thousands of monasteries and temples were destroyed during the Cultural Revolution from 1966 to 1976.

A French traveler who arrived in Beijing from Lhasa tonight and asked not to be named, said Tibetans had thrust stones at foreigners during the demonstration, urging them to throw them at police.

Other western travelers, also arriving in Beijing from Lhasa tonight, said the Jokhang Temple reopened in an atmosphere of religious fervor this morning for the first time since Thursday's riot.

A tourist said the government office that issues travel permits for foreign tourists to go to areas outside Lhasa had been closed since the Thursday riot.