Argentina launched a new crackdown on capital flight, tax evasion and money laundering Monday, requiring people buying dollars to first prove they're up to date on their taxes.

Flexing its enforcement power only days after winning re-election by a landslide, the government says that anyone trading Argentine pesos for foreign currencies must first show where they got the money, based on wealth or income previously declared to the tax agency.

That's only a paperwork problem for people and businesses that are fully compliant on their taxes.

But tax evasion runs rampant in Argentina, as in many countries in the developing world, with an estimated 34 percent of the Argentine economy operating in undeclared cash. Many of these scofflaws are expected to turn even more to the black market for their dollars, but the government is cracking down there as well, arresting operators who have long facilitated illegal money flows.

Many Argentines were anxious Monday as the country adjusted to the new rules announced Friday night. Currency exchange houses in downtown Buenos Aires were mostly empty, save for a few tourists buying pesos with their foreign currencies. Outside some of these businesses, signs showed only the purchase price, suggesting that for the moment, they weren't selling dollars at any price.

Complying with the new measures shouldn't take any longer than the confirmation of a purchase using a credit card, Economy Minister Amado Boudou said Monday. People wanting dollars simply need to enter their national identity number and tax number into the database of AFIP, Argentina's tax agency, and will promptly get approved or rejected based on how the amount involved matches the person's income and wealth, he said.

Boudou also urged Argentines to trust that their government won't allow the currency to drastically fall from its current rate of 4.26 pesos to the dollar.

But for many people in the capital, it was all but impossible to buy dollars Monday. Some exchange houses posted signs saying all currency trading was suspended while their systems were being updated, and people applying online to get approval for even nominal sums were rejected. One account representative for a major multinational bank said he had applied to buy relatively small amounts of dollars on behalf of eight clients, and got only two responses so far, both negative.

"It's really slow, more than an hour's wait," the banker said, speaking on condition of anonymity due to the sensitivity of the topic.

Economist Fausto Spotorno predicted that the currency controls will only sap more confidence from the Argentine economy and prompt more people to hide their money from the government. This could increase capital outflows, which are already estimated at $3 billion a month, as more people turn to the black market, he warned.

Last week, President Cristina Fernandez imposed new requirements on oil and mining and insurance companies, ending exemptions that enabled them to convert most of their Argentine profits into foreign currencies outside the country. From now on, these companies, like nearly all others, must fully declare their profits within Argentina's tax and currency system, increasing the local money supply by as much as $4 billion, analyst Daniel Kerner said.

Fernandez also announced that insurance companies must make all their investments inside Argentina. Before, they could invest 15 percent outside the country. This could generate as much as $2 billion more, Kerner said.

Argentina's modern history has been so marked by catastrophic political and economic upheavals that many citizens are convinced another disaster is just around the bend. That's a big reason why they have stashed about $160 billion of their wealth outside the country. That's nearly half the size of the country's entire economy, which generated $370 billion in GDP last year.

With relatively low foreign debt (15 percent of GDP) and a favorable trade balance, Argentina's government has been able to live with the capital outflows, spending dollars from its foreign reserves to maintain the peso's value.

But with inflation running as high as 25 percent a year, according to private estimates, the demand for dollars is becoming ever stronger. The Central Bank had to sell $2 billion in dollar reserves in October, nearly $500 million more than the month before. Even with significant inflows from dollar-denominated commodities exports, the bank's foreign reserves fell from a peak of about $52.6 billion in January to $47.6 billion on Friday, after last week's election.

Argentina had just $11 billion in foreign reserves when Fernandez's late husband and predecessor, Nestor Kirchner, took office in 2003. The country was only beginning to bounce back from its world-record default and currency devaluation back then, and the government had little wealth to share.

Now Fernandez is desperate for dollars because social spending has risen to an unsustainable $27.8 billion this year, according to economist Orlando J. Ferreres, who runs his own mergers and acquisitions firm in Argentina.

But Boudou stressed that the laws haven't changed; they're just being enforced like never before.

Evading taxes was easier in Argentina before the government updated its computer systems. Now, the tax agency, Central Bank and money laundering watchdog all have access to the same data, and can detect when someone is moving more cash than can be justified by their declared income.

"Nothing has changed with respect to the possibility of buying dollars. Everyone who can demonstrate their income can acquire them," Boudou, now doubling as vice president-elect, said Monday on Twitter.

Then he added a warning: "But those who hoped that this government would stop defending the country because it won the elections, forget about it. This government works for Argentina."

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Almudena Calatrava in Buenos Aires contributed to this story.

Michael Warren can be reached at http://www.twitter.com/mwarrenap

(This version CORRECTS the peak reserve figure to $52.6 billion in January. )