Police in the Nigerian capital Abuja have used tear-gas to disperse about 200 demonstrators protesting against the removal of government subsidies on fuel.
Fuel prices have more than doubled in many areas since the decision came into force on Monday.
The government has argued that ending the $6 billion subsidy will free up money for programmes aimed at alleviating poverty.
Announcing the end of the subsidy, the government urged people not to panic-buy or hoard fuel.
Nigeria's two main labour organisations, the Trades Union Congress and the Nigerian Labour Congress, issued a joint statement condemning the move.
Nigeria is Africa's biggest oil producer, but imports refined petrol.
The BBC reports that years of mismanagement and corruption mean it does not have the capacity to refine oil, turning it into petrol and other fuels.
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 $8 billion a year.