By Charles Haviland
BBC News, Kathmandu
One of the schoolchildren inside the UN compound
A group of more than 20 Tibetan schoolchildren has staged a protest in the main United Nations compound in Nepal's capital, Kathmandu.
A UN spokesman said the children, aged between 15 and 18, and dressed in school uniform scaled the wall.
This is the boldest Tibetan protest in nearly three weeks in Kathmandu.
Tibetan exiles say police have also arrested nearly 90 of their community as their demonstrations against Chinese rule in Tibet continue.
A UN spokesman told the BBC the 21 students apologised to senior officials and guards for entering the UN compound.
The children said they wanted to draw attention to the crisis in Tibet and unfurled a flag featuring independence slogans for the Chinese-ruled region, the spokesman said.
Nepal says it cannot allow Tibetans to demonstrate
He said the UN resident co-ordinator had one prime concern - that no charges be levelled against the children.
He had asked the authorities not to allow the use of force against Tibetan demonstrators.
As the students climbed into the UN, the police attention appears to have been diverted by a bigger demonstration nearby, from which they arrested 89 people, say Tibetan activists.
Kathmandu is home to thousands of Tibetan exiles and over the past fortnight they have mounted almost daily protests against Beijing.
Several hundred of the demonstrators have been detained.
Meanwhile, Amnesty International and the UN have both criticised the government's handling of the protests.
Protestors have been beaten with considerable force
On Monday the UN Office of the High Commissioner for Human Rights in Nepal said the Nepalese police were arresting people who were not demonstrating purely on the basis of their appearance - an apparent reference to Tibetans.
The police have been filmed beating protesters, including monks, with considerable force.
Nepal says it cannot allow Tibetans to demonstrate because it recognises Tibet as an integral part of China.
But the UN says the mass arrests are against the spirit of a society governed by the rule of law.