By Drew Hinshaw and Drew Hinshaw  |  WSJ

With 187 million people, and trillions of dollars in untapped crude oil, Nigeria was meant to power Africa’s rise. Instead, it is becoming—for the moment—a symbol of how fast and far low oil prices have dragged emerging markets down.

Months of dwindling oil revenue have prompted a scarcity of dollars here, as the government hoards foreign currency to safeguard shrinking reserves. That is starting to hit Nigerians rich and poor alike: On Monday, the country’s stock market fell almost 3% on news that MSCI is considering removing the country from its benchmark frontier markets index.

Meanwhile, the World Bank said Nigeria’s economic growth slid to 2.8% in 2015 from 6.3% the year before, and the International Monetary Fund says this year’s growth will slip to 2.3%, slower than the population, which adds 13,000 people daily.

Factories are closing because they can’t find dollars to import parts. Supermarkets are struggling to keep shelves stocked. Power plants have virtually stopped producing electricity because they can’t pay for maintenance. New shopping malls are empty and ordinary citizens are going to lengths to find some basic goods.

To keep his economy growing, President Muhammadu Buhari is traveling to China this week, hoping to secure a multibillion-dollar loan for new infrastructure, including railroads, spokesman Garba Shehu said. This year, Nigeria may issue its first yuan-denominated bond, Finance Minister Kemi Adeosun said on Saturday.

Back home, Africa’s top oil producer is unable to import enough gasoline. Drivers in this city of 21 million have spent days inching through miles-long lines to fill their tanks at the few pumps still operating. To keep order, soldiers snap whips at oil can-toting line-jumpers and break up fights between exasperated drivers.

“We are hungry and angry,” said Victor Eten, a taxi driver who slept in his cab for three days to buy gas. “No shower, no toothbrush.…If this continues, there will be big trouble.”

Until recently, Nigeria and its economic capital were symbols of Africa’s new consumer class. Cineplexes, car dealerships and a fast-food arms race—KFC and Domino’s, among others, opened here—spoke to the aspirations of the continent’s largest city, Lagos. A decade of 7% economic growth brought Nigeria close to entering the world’s 20 largest economies. It also lured home Nigerian talent from jobs and schools in the U.S. and Europe.

These days, the euphoria has dimmed in Africa’s most populous nation. The government, which sees the downturn as an opportunity to industrialize—breaking Nigeria’s dependence on imports in an economy that relies on oil for three-quarters of revenue—also concedes that its constituents could face years of pain.

“It will take a minimum of 18 months before we begin to see a recovery,” said presidential spokesman Femi Adesina. “Through deft economic engineering, things will bounce back, but it’s not going to be magic. It’s not going to be overnight.”

Mr. Buhari has made progress in beating back the jihadist insurgency Boko Haram since taking office in May. Soldiers now hold down towns and highways once controlled by the Islamist group, whose violence occurs far from the country’s economic nerve center.

He is also making moves against corruption: Each day at 3 p.m., the new finance minister calls a different government agency and combs through its expenditures, item by item.

But on the streets, daily frustrations are mounting. Electricity is so scarce that the country’s national power plants didn’t produce a single watt for several days last week—they couldn’t import parts and services, said two senior members of Mr. Buhari’s administration. Internet providers face similar woes.

Nigerians abroad are stuck with ATM cards they can’t use because the central bank has limited withdrawals outside the country. Bitcoin trades are up as Nigerian professionals scrounge for ways to move money—and increasingly, themselves—out of the country.

“The structural worry I see is the middle class,” said Keith Richards, a Guinness executive who has worked in Nigeria for four decades. “We could see an exodus of the future of this country. People are already leaving.”

Mr. Buhari says he hopes the scarcity of foreign goods will lead Nigerians to buy from their own farms and factories, sparking an industrial renaissance. Many new regulations encourage people to use Nigerian steel, eat local rice, and spend within the country’s borders. To show his commitment, the central bank governor recently buried his mother in a made-in-Nigeria funeral, with food, drink and decorations all sourced locally: “The central bank governor practiced what he preached,” said one senior bank official.

Civil servants have been particularly hit: Mr. Buhari says his government inherited an empty treasury after crude prices collapsed starting in 2014. Twenty-seven of the country’s 36 states are struggling to pay civil servants, he has said. He has asked lawmakers to cut spending, but they have balked, leaving the president without a budget he is willing to sign.

Revenue recently took a second hit when saboteurs went underwater to break open a pipeline that carries 130,000 barrels of crude a day. The government says that attack, which it sees as a political move to undermine the president, cost the state $122 million in February alone.

Aggravating the oil shock is a mounting foreign-currency crisis. In a bid to defend the naira, Nigeria’s central bank has sharply restricted the availability of dollars. A weekly committee stipulates which banks are allowed to sell dollars and to whom, for what purchases, and at what price.

The result: Businesses are increasingly unable to get the foreign exchange they need to import spare parts, pay off foreign lenders, travel internationally, and keep the economy running. Nigerians entrepreneurial enough to find dollars sell them for as much as twice the official exchange rate in back-alley trading shops, restaurants and on the street.

Mr. Buhari’s administration, which rode into power in May 2015 with a pledge to oust corruption, is running out of political room to act. Last year, his supporters danced at rallies waving his campaign logo—a broom—signifying a clean start. In recent months, newspapers have run stories about disenchanted voters burning brooms in bonfires.

“They’ve got to get started,” said Bismarck Rewane, managing director of Lagos research firm Financial Derivatives Co. “People are getting a bit impatient. That means there has to be action.…There’s some tension growing.”

From her laptop emporium in a four-story Lagos mall, saleswoman Joyce Nwando has watched young professionals who were meant to power her country’s rise vanish. A year ago, selfie-snapping shoppers packed the food court: “It used to be that everything that happens in Lagos happens here,” she said. Now, many stores are vacant, the lights are often off and some shopkeepers say they may close in the next few months. On Friday, a 3-D theater upstairs was about to screen the debut of “Batman v. Superman.” Not a single customer was there. “People really don’t have the money,” said the theater’s general manager, Franson Davis. “Everybody is just waiting for the light at the end of the tunnel.”

Write to Drew Hinshaw at drew.hinshaw@wsj.com and Joe Parkinson at joe.parkinson@wsj.com