Thousands of Nigerians are taking part in protests around the country following the removal of a fuel subsidy, which has doubled petrol prices and transport fares.
Trade unionists have marched in the biggest city, Lagos. They 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 into 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 protests
Tuesday is the first working day since the measures were announced on 1 January.
Protests have been held in cities across the country, including Lagos, Ibadan, Lokoja, Nasarawa and Kano.
The BBC's Tomi Oladipo in Lagos says that as the marchers passed petrol stations, they shouted insults and tried to persuade them to close.
They also disrupted traffic on a major highway and started fires.
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.
On Monday, police fired tear gas to disperse some 200 people who had gathered in the capital, chanting: "Remove corruption, not subsidy."
Prices have increased from 65 naira ($0.40; £0.26) per litre to at least 140 naira in filling stations and from 100 naira to at least 200 on the black market, where many Nigerians buy their fuel.
There are reports that petrol prices have tripled in some remote areas, while commuters have complained that motorcycle and minibus taxi fares have already doubled or tripled.
Many Nigerians expect the prices of other goods to rise as well.
The government has said it will spend the money saved by removing the subsidy on improving the country's erratic electricity supply, as well as health and education.
However, analysts say that many Nigerians have little faith that the money will be well spent and fear it will instead be stolen by corrupt officials.
In December, the government released a list of the people who benefit most from the subsidy, which include some of Nigeria's richest people - the owners of fuel-importing firms.