Nigerians took to the streets across the country for a fourth day running on Thursday to protest against the government's decision to remove popular fuel subsidies, which more than doubled the price of petrol.

As tens of thousands joined demonstrations, unions showed no signs on backing down on their threat to paralyse the country with an indefinite strike and shut down oil production in Africa's top crude producer.

Industry officials doubted unions would be able to stop crude exports completely, but even a minor outage could have a significant impact on the economy.

Protests began again in cities everywhere from the commercial centre Lagos in the south to the remote and restive city of Maiduguri in the far northeast.

Banks, shops, airports and schools remained closed but small market sellers and roadside hawkers carried on trading.

We're willing to carry on the strike for as long as it takes for the government to rescind its decision on the removal of the subsidy, one protester in Maiduguri said.

The January 1 removal of the subsidy sent the pump price to 150 naira ($0.93) a litre from 65 naira overnight.

Crowds in Lagos, Nigeria's main commercial city, chanted anti-government slogans and sang solidarity anthems. Some protesters said things could get worse if a resolution was not found soon.

We might change our strategy as the next level of protests starts, one demonstrator said in Lagos. So far the protests have been largely peaceful.

Nigeria's biggest oil union said on Wednesday it was ready to halt oil output if the government did not reinstate the subsidy.

Africa's largest oil producer relies on crude exports for more than 90 percent of its foreign exchange earnings. Even a minor outage would put pressure on President Goodluck Jonathan and his team to take action against protesters or negotiate.

If there is any disruption to oil production it would be a serious escalation and the government would be likely to use legal or enforcement means to stop it, said Kayode Akindele, partner at Lagos-based investment firm 46 Parallels.

Jonathan has so far shown no sign of bowing to the protests.

OIL MONEY

Nigeria exports around 2 million barrels of crude oil per day and is a key supplier to the United States and Europe. Output has been unaffected so far but oil traders said concerns about disruptions to Nigerian output helped global prices rise more than 1 percent on Thursday.

We believe that a government that is alive to its responsibilities will not allow this strike to degenerate thus far ... we hereby direct all production platforms to be on red alert in preparation for total production shutdown, said a statement from the oil union PENGASSAN.

Information Minister Labaran Maku told Reuters the government was worried about the threat and wanted to keep negotiating with unions. But industry officials said Nigeria had oil in storage and they doubted unions could shut down crude exports completely.

Nigeria's main union remained defiant, insisting it would continue its strike until fuel price subsidies were reinstated.

We are certainly continuing with protests (Thursday). We won't surrender until the pump price of petrol is reverted to 65 naira, Owei Lakemfa, general secretary of the National Labour Congress, told Reuters by phone in Abuja.

Economists say the subsidy fuelled corruption and keeping it in place would have forced Nigeria into huge external borrowing, but most Nigerians, who live on less than $2 a day, saw it as their most tangible welfare benefit.

Despite holding the world's seventh-largest gas reserves and producing over 2 million bpd of crude, decades of graft and mismanagement mean Nigeria doesn't refine its own fuel.

Jonathan is now facing two major security headaches -- opposition to the fuel price spike and almost daily attacks buy an Islamist group in the north.

The leader of Boko Haram, which wants Islamic sharia law more widely applied across Nigeria, appeared in an online video on Tuesday saying Jonathan did not have the capability to stop the group's insurgency.

(Additional reporting by Camillus Eboh in Abuja, Ibrahim Mshelizza in Maiduguri; Editing by Andrew Heavens)