The actor Jackie Chan joined Chinese officials and other celebrities today in an extravagant celebration of the 100 days countdown to Beijing’s Summer Olympics.

Thousands gathered in the capital to sing “Beijing welcomes you", a song written especially for the countdown. Waves of dancers performed with their hair styled like Olympic rings.

But the grand ceremony was marred by criticism of China from both the White House and Gordon Brown. The Prime Minister said today he was "unhappy" about the country’s actions in Tibet.

The White House said it was “concerned” about the decision of a Chinese Court to jail 30 Tibetans after they rioted in their capital, Lhasa, last month.

Hundreds of Tibetans rampaged through Lhasa's streets on March 17, stabbing and stoning ethnic Han Chinese and setting fire to hundreds of shops and offices.

Officials said that 18 people were killed.

The Intermediate People’s Court in Lhasa yesterday handed down terms of three years to life to the 30 protesters. A Buddhist monk was sentenced to life in prison.

Dana Perino, the White House spokeswoman, said the President was "concerned" about the sentences. “We don’t think that anyone should break the law," she said. "But we also believe in freedom of expression and assembly.”

China has blamed Tibet’s spiritual leader, the Dalai Lama, and his government-in-exile for plotting the riots.

This week it sealed off the Tibetan capital before the arrival of the Olympic torch relay, closing the Himalayan city to farmers and insisting that even schoolchildren must carry new passes.

Panic buying gripped the city as hundreds of thousands of people prepare for the tight security restrictions.

The International Campaign for Tibet said today that groups of Buddhist monks have been detained from several Lhasa monasteries, which have been sealed off by troops.