The authorities in Nigeria have started knocking down slum dwellings built on the lagoon in its biggest city Lagos.Dozens of shacks built on stilts have been demolished in Makoko, where wooden canoes are a common form of transport.The BBC's Will Ross in Lagos says it is not clear whether all houses in the area are to be destroyed.Makoko is one of Nigeria's best known slums. Many residents are fishermen and some have migrated from neighbouring Togo and Benin.It featured in the 2010 BBC film Welcome to Lagos, which angered the Nigerian government. It accused the film-makers of showing Nigeria in a negative light.MachetesA letter was served on residents last week, giving them 72 hours to vacate their properties.Several told the BBC they did not know where they and their families would sleep.Our correspondent saw men using machetes to chop down the stilts of the wooden homes, while police watched from nearby boats.The letter from the Lagos state authorities says the illegal constructions constituted an "environmental nuisance, security risk and an impediment to the economic and gainful utilisation of the waterfront" and undermined the "megacity status" of Lagos.The authorities have not said how many people will be affected but community leaders say tens of thousands of people live in Makoko, the AFP news agency reports.The slum is easily visible from the bridge which connects the Nigerian mainland to the city's rich island districts.Our correspondent says the slum destruction is part of efforts to clean up Lagos.State governor Babatunde Raji Fashola says he wants to get the city ready for its predicted population of 40 million people.The city is building a light railway and has widened the roads, easing the city's once notorious traffic jams.