Hong Kong\'s biggest teachers\' union on Wednesday called on city officials to block a proposal to usher in mandatory classes about Chinese patriotism. Earlier this year, the city\'s top education officials proposed making the controversial classes compulsory for school children, sparking fears of political brainwashing. James Hon, a spokesman for the Hong Kong Professional Teachers\' Union, rejected the proposal outright. \"This is clearly political interference (in our education system),\" he said, adding that it would be \"detrimental to students\' all-rounded development\". The former British colony, which returned to China in 1997, is a semi-autonomous territory which maintains its own political and legal system, with civil liberties not seen on the mainland. Protests against Beijing are a regular fixture in the city, which has a population of seven million. Under the proposal students would take 50 hours of lessons a year focusing on \"building national harmony, identity and unity among individuals\". There would be no exams, but classes would assess if pupils \"feel happy to be Chinese\" or \"consider the needs of the country when planning their future\", according to guidelines posted on the website of the Education Bureau. Advocates said it would help students \"develop a sense of belonging to the motherland\", \"support national sports teams\" and \"appreciate Chinese culture\". The union, which has a membership of 80,000 -- 90 percent of the city\'s teachers -- recently released a survey which found 70 percent of its members opposed the classes. The survey showed 67 percent were \"worried\" or \"very worried\" the subject could brainwash students. If approved, the classes will be implemented in primary schools starting next year and in secondary schools from 2013.