Fears of more bloodshed in Tibet are growing after an ultimatum imposed by the Chinese authorities ran out.Rioters in the capital Lhasa - who are demanding an end to rule from Beijing - were given until midnight local time to surrender.Tibetan students are set to stage a hunger strike outside the Chinese embassy in central London to protest against what they say is a violent backlash against the growing independence movement by China's army.The Chinese authorities have warned they will "deal harshly" with anyone in Lhasa who defied their orders and a huge military convoy was reported to be heading for the Tibetan border.Tibet's exiled spiritual leader, the Dalai Lama, is concerned about more clashes and has warned of "cultural genocide".But other members of the Himalayan region's exiled government expressed frustration at the Buddhist leader's insistence on a non-violent "middle way".Tibetan groups say 80 people have been killed in the increasingly violent clashes.China has said at 13 "innocent civilians" died, most of them in fires lit by rioters in Lhasa, but troops did not use guns on protestors.At Dharamsala in northern India, the home of the Dalai Lama, Tibetan exiles have been holding candlelit vigils for the demonstrators and burning Chinese flags.And for the first time, protests at China's treatment of the so-called "autonomous region" spread to Beijing, with dozens of students staging a sit-in at the capital's Central University for Nationalities.Britain's Prime Minister Gordon Brown called for the Chinese to show "restraint" and said: "The whole world, at this time, is looking to China to see what the reaction is."Beijing has defended the crackdown, saying police acted lawfully to stop rioting after a week of demonstrations led by Buddhist monks spilled over into violence.China is keen to present a trouble-free image of unity ahead of the Beijing Games in August this year.© Independent Television News Limited 2008. All rights reserved.