At least 80 people have been killed in unrest following protests by Tibetans against Chinese rule, the Tibetan government in exile says.
Chinese troops were out in force in Lhasa on Sunday
Indian-based officials said the figure was confirmed by several sources, even though China put the death toll at 10.
The Dalai Lama called for an international inquiry into China's crackdown, accusing it of a "rule of terror" and "cultural genocide".
Chinese troops were out in force in Lhasa, Tibet's main city, on Sunday.
Hong Kong Cable TV reported that about 200 military vehicles, each carrying 40 to 60 armed soldiers, had driven into the city.
Loudspeakers broadcast messages, such as: "Discern between enemies and friends, maintain order."
China tightly restricts Western journalists' access to Tibet and it is sometimes extremely difficult to verify what is going on.
The BBC has learned that troops in neighbouring Sichuan province have been recalled from leave and put on standby.
China says Tibet always part of its territory
Tibet enjoyed long periods of autonomy before 20th century
1950: China launched a military assault
Opposition to Chinese rule led to bloody uprising in 1959
Tibet's spiritual leader the Dalai Lama fled to India
A 23-year-old Canadian student in Lhasa told AP: "The entire city is basically closed down."
The Chinese crackdown followed rioting on Friday, that erupted after a week of mainly peaceful protests.
The Chinese official news agency Xinhua says 10 people died on Friday, including business people it said were "burnt to death".
But the Tibetan government in exile later said at least 80 corpses had been counted, including those of 26 people killed on Saturday next to the Dratchi prison in Lhasa.
Other bodies were spotted near the Ramoche Buddhist temple, and near a Muslim mosque and a cathedral in Lhasa, said Tenzin Taklha, a senior aide to the Dalai Lama.
"These reports come from relatives, from our people inside and from contacts of our department of security. They have all been confirmed multiple times," he said.
Deadline to surrender
The demonstrators, who on Friday set fire to Chinese-owed shops and hurled rocks at local police, have been penned into an area of the old town by government forces.
The authorities in Tibet have urged the protesters to hand themselves in by midnight on Monday, promising leniency to those who surrender.
Meanwhile, there were reports of protests by Tibetans in other parts of China.
About 200 protesters threw petrol bombs and burned down a police station in Sichuan province, a police officer told Reuters.
There were reports that officers opened fire on the protesters.
In an interview with the BBC, the Dalai Lama, the Tibetan spiritual leader, said he feared there would be more deaths unless Beijing changed its policies towards Tibet, which it has ruled since invading in 1950.
"It has become really very, very tense. Now today and yesterday, the Tibetan side is determined. The Chinese side also equally determined. So that means, the result: killing, more suffering," he said.
"Ultimately, the Chinese government is clinging of policy, not looking at the reality. They simply feel they have gun - so they can control. Obviously they can control. But they cannot control human mind," he warned.
The unrest erupted a fortnight before China's Olympic celebrations kick off with the start of the torch relay, which is scheduled to pass through Tibet.
But the Dalai Lama emphasised that he still supported Beijing's staging of the Olympic Games this summer, saying it was an opportunity for the Chinese to show their support for the principle of freedom.
The International Olympic Committee said it hoped to see the Tibetan unrest resolved peacefully, but its president Jacques Rogge rejected any boycott, saying it "doesn't solve anything".
US Secretary of State Condoleezza Rice on Saturday urged China to "exercise restraint" in dealing with the protests.
She spoke as pro-Tibet demonstrations were held in Nepal, New York, Australia and several European cities.