Hundreds of Nigerians are marching in the commercial capital, Lagos, to protest at the removal of a fuel subsidy, which has led petrol prices to double.They left from the headquarters of main trade unions, which are to meet on Wednesday to decide on strike plans.About two dozen people have been arrested in the main northern city of Kano as they gathered to protest.Nigeria is Africa's biggest oil producer, but imports refined petrol.Years of mismanagement and corruption mean it does not have the capacity to refine oil, turning it into petrol and other fuels.Analysts say many Nigerians regard cheap fuel as the only benefit they get from the nation's oil wealth.Several previous governments have tried to remove the subsidy but have backed down in the face of widespread public protests and reduced it instead.The IMF has long urged Nigeria's government to remove the subsidy, which costs a reported $8bn (£5.2bn) a year.Police block protestsTuesday is the first working day since the measures were announced on 1 January.The BBC's Tomi Oladipo in Lagos says that as the marchers pass petrol stations, they shout insults and try to persuade them to close.They have also disrupted traffic on a major highway and started fires.He says that it is not yet clear where the marchers are heading.The city is not as busy as it normally is, with many people stranded as they cannot afford the new transport fares after going away for the holidays, our reporter says.Police in the capital, Abuja have blockaded Eagle Square, where protesters had planned to gather for a protest.Tuesday is the first working day since the measures were announced on 1 January.The BBC's Tomi Oladipo in Lagos says that as the marchers pass petrol stations, they shout insults and try to persuade them to close.They have also disrupted traffic on a major highway and started fires.He says that it is not yet clear where the marchers are heading.The city is not as busy as it normally is, with many people stranded as they cannot afford the new transport fares after going away for the holidays, our reporter says.Police in the capital, Abuja have blockaded Eagle Square, where protesters had planned to gather for a protest.