Nepal has agreed to a Chinese request to temporarily block access to Mount Everest, amid fears Tibetan activists may stage a protest at the peak.
Spring is said to be the best time to scale Mount Everest
Climbers will be banned from going beyond Everest's base camp until 10 May, Nepal's tourism minister said.
Beijing is thought to be concerned that a protest may coincide with plans to take the Olympic torch up the mountain.
Tibetans have stepped up agitation against Chinese rule as Beijing gets ready to host the Olympics in August.
Buddhist monks have been demonstrating in and around Lhasa, the capital of the Tibet region.
Many Tibetans believe their region ought to be independent, though China has long claimed the mountainous territory as its own.
Meanwhile police in the Nepalese capital, Kathmandu, have used force to break up a gathering of Tibetan refugees for the second time this week.
Tibetan eyewitnesses say that nuns were among those beaten with sticks.
Nepalese Tourism Minister Prithvi Subba Gurung told Reuters news agency that climbers would not be allowed beyond Mount Everest's base camp until 10 May.
He said the move was in response to a request from China.
"This is to prevent some people who could infiltrate and cause trouble during the time when they take the torch to the top."
Police and Tibetan exiles clash in Kathmandu on Friday
No exact date has been set for the Olympic torch's journey to Mount Everest but reports say it is likely to be in late April or early May.
May is widely regarded as the best time of year to climb the peak, the world's highest at 8,848 metres (29,028 ft).
Mount Everest straddles the border between Nepal and China and can be scaled by a northern and a southern route. The more popular southern route passes through Nepal, while the northern ascent goes through China's Tibet region.
China was reported earlier this week to have restricted access along the northern route, amid fears that pro-Tibetan activists may disrupt the Olympic torch's journey.
Last year, five US mountaineers, including an ethnic Tibetan, unfurled banners on Everest calling for Tibetan independence.
No Nepal compromise
In the latest violence in the Nepalese capital, a policeman who declined to give his name told the BBC that members of the Tibetan refugee community had staged a candlelit rally at the main Tibetan Buddhist shrine in Kathmandu.
He said they had been chanting slogans such as "Free Tibet" and had sworn at the police and then thrown stones and bottles, so the police charged them with batons in retaliation.
Two Tibetan men, however, told the BBC there had been no rally and that the police had beaten hundreds of people including nuns who had been lighting candles and saying prayers.
One of the men said his hand had been broken in a police beating. Some of the Tibetans were detained for a few hours.
On Monday the police beat back hundreds of Tibetans who were trying to march to the Chinese embassy.
The BBC's Charles Haviland in Kathmandu says Nepal has been showing it will not compromise on its support for its huge neighbour, China, on issues including Tibet.
In India, the Tibetans' spiritual leader, the Dalai Lama, has expressed concern at China's response to the protests in Lhasa.
The unrest began last week, as Tibetans marked the 49th anniversary of the Dalai Lama's journey into exile from Lhasa to India.
Some 100 protesters were detained by Indian police on Thursday in an attempt to halt a planned march to the Chinese border.
The protesters in the northern Indian state of Himachal Pradesh will be held in custody for two weeks, police said.
US-based pressure group Human Rights Watch has criticised the authorities' response to the protests.