Protests broke out in at least two Tibetan-populated towns in Gansu this week as middle-school students took to the streets on the anniversary of deadly rioting two years ago. The protests took place despite tight security in Tibetan-populated areas ahead of the March 14 anniversary. At least 20 slogan-chanting students from Machu county's Machu Tibetan Secondary School marched to the county government headquarters on Sunday, a witness said yesterday. He said the students waved snow lion flags as they protested. The flag is used by the Tibetan government in exile in India but is outlawed by mainland authorities. 'They chanted slogans in Tibetan that I don't understand and two of them also waved a banner with Tibetan characters,' he said. He said the student protest, which started out from the school at about 1pm, drew the attention of crowds on the streets and left many non-Tibetan vendors and businessmen scrambling to close their shops. 'The students were tailed by police cars. But the police just followed, they didn't do anything,' he said. 'Many shops are still closed today as some people are still worried by the protests.' Radio Free Asia quoted Machu residents as saying the student shouted slogans decrying the lack of freedom and calling for Tibetan independence. It said the protest was joined by 500 to 600 other Tibetans. The report said at least 40 people were detained by police on Sunday, prompting a fresh round of protest as hundreds of Tibetans demanded the release of the detainees. The man who witnessed the protest said security had tightened up since, with almost all the county's police officers patrolling the streets. 'I heard that officials have been sent to rural areas and schools to talk to the people as a preventive measure,' he said. The Machu police and secondary school refused to comment yesterday on the protest. A similar protest led by teenagers broke out in nearby Hezuo on Sunday, Reuters reported yesterday. The report quoted a hotel employee as saying that at least 20 teenagers were taken in custody by police on Wednesday. March has become a sensitive period for the central government after the pro-independence unrest in 2008 claimed hundreds of lives and prompted international criticism of China's Tibet policies that later turned into calls for a boycott of the Beijing Olympics. The unrest began in Lhasa , capital of the Tibet Autonomous Region , and later spread to other Tibetan areas including Machu. The unrest was sparked by a protest on March 10 that year to mark the anniversary of the failed uprising in 1959 that resulted in the Tibetan spiritual leader the Dalai Lama and his administration fleeing to India. A Tibetan tour guide in Lhasa said yesterday that foreign tourists had been barred from entering Tibet this month and security in the city had been boosted for the anniversary. Beijing has adopted a pump-priming approach in the Himalayan region in an attempt to stifle criticism of a lack of religious freedom and freedom of speech and other human rights violations. But critics say the unrest in 2008 showed that central government policies had failed to address Tibetans' concerns and needs. At a high-level meeting this year to discuss Beijing's development plan for Tibet in the next decade, the central government renewed its pledge of more investment in the autonomous region and other Tibetan-populated areas.