PARIS » There was nothing extraordinary about the casually dressed businessman waiting on a Paris train platform except, it turned out, for the envelopes he carried – stuffed with 350,000 euros in cash, and seized by French customs agents as he prepared to depart for Belgium.

The passenger, Boris Boillon, 43, is a former French ambassador to Iraq and Tunisia with two degrees from prestigious universities and a Legion of Honor medal. But to customs agents, who seized the money in July, he was just one of a growing number of "cash commuters."

At the borders of European countries in economic crisis, customs agents say they are seizing increasing amounts of undeclared cash exceeding the 10,000 euros ($13,750) that each traveler is allowed to carry. They find it stashed in luggage, cake boxes, potato-chip bags, cookie tins and sometimes even children’s pockets. The cash, often in bundles of 500-euro notes, is moving with political currents as some Europeans seek to hide their wealth from rising taxes, high-profile tax investigations and tightening rules at Swiss banks and other traditional havens.

The agents say they are routinely detaining business travelers who are on their way to European financial capitals, carrying minimal luggage and behaving nervously. "We see professionals and businessmen in insurance and banking, like him, every day," said Philippe Bock, secretary general of the French solidarity trade union for customs agents, referring to Boillon.

"Three hundred fifty thousand euro was nothing exceptional," Bock said. "Every month it passes like that, and there’s more and more money because of the crisis."

For decades, banking secrecy laws in Switzerland made banks there a refuge for foreigners hoping to keep assets away from official notice. But Switzerland signed a treaty in October providing for the automatic exchange of tax information with depositors’ home countries, and bankers have been warning clients to make tax declarations or risk having their Swiss accounts closed. That has left many would-be tax avoiders with little choice but to move their money around the old-fashioned way.

"The main reason for the increase in seizures is simply the rising use of cash by fraudsters, including criminal networks and tax evaders," said Mathieu Delahousse, a French journalist and co-author of a book about the phenomenon, "Cache Cash." "People are still taking money abroad for tax evasion, but it is also moving in the other direction, because Swiss banks are closing accounts of foreign customers, and then they have to make a choice: declare these bank accounts and pay high taxes, or hide the money."

The rule requiring travelers crossing borders within the European Union to make a written customs declaration when carrying more than 10,000 euros in cash was introduced in 2007 in the hope of deterring money laundering and tax evasion. Undeclared cash can be seized and held for six months, and fines of 25 percent or more can be withheld. The authorities can also start broader investigations into the origins of the money in special customs courts.

Customs agents are blasi about catching business people, but they were amused to see an American family of four, including two young children, in a discreet corner of a railway station near the Swiss border, dividing 600,000 euros among themselves. The money was seized when they boarded a train, according to Bock of the customs trade union.

Cash seizures by French customs agents have soared over the last decade even as budget cuts have thinned the agents’ ranks by 25 percent. The total for the first quarter of 2013 was up sixfold from a year earlier, to 103 million euros, most of it from a man who tried to drive into France from Switzerland with 86 million euros in bearer bonds, which are tantamount to cash. On an average day in 2012, French agents seized 300,000 euros, 50 percent more than the 2011 average, according to government figures. And the customs agency estimates that it catches only 5 percent of the undeclared cash crossing the country’s borders.

The precautions are growing more elaborate, and the finds more eye-catching. In February, inspectors on the fast train between Zurich and Paris stopped a Spanish traveler who was carrying 1.8 million euros ($2.5 million), made up entirely of 500-euro notes. Those bills, the largest denomination in circulation, have come to be nicknamed Bin Ladens for their association with money laundering and illicit transactions.

In Italy, cash-sniffing Labrador retrievers and German shepherds helped the financial police who prowl the country’s five main airports and its northern border to nearly triple their seizures of cash in 2012, to 124 million euros. The authorities were close to exceeding that figure this year by mid-autumn.

Sergio Callipo, national secretary of the Italian customs agents’ union, said that much more cash slipped through. "Millions of passengers pass every week, and some officers and one or two dogs are not a real deterrent for smugglers," he said. "We would need an army, and we are just sentinels."

Some experts believe that widespread publicity about tax investigations of well-known figures in struggling countries is driving an increase in cash smuggling. Those ensnared include Jirtme Cahuzac, the former French budget minister; Uli Hoeness, the president of the Bayern Munich soccer club in Germany; and the soccer star Lionel Messi of FC Barcelona in Spain.

Some wealthy travelers complain that they feel like targets. A European aristocrat who did not want to be named, for fear of drawing more scrutiny, said that friends were sharing stories of Italian financial police boarding docked yachts to check for undeclared cash in safes on board.

Like their French counterparts, Spanish customs agents and tax authorities say they have been making more seizures this year despite their ranks’ having been depleted by budget cuts. In Spain, most seizures are of undeclared money leaving the country, but some are of cash on its way in.

The bulk of the Spanish cash seizures are in euros – about 17.5 million euros so far this year – but some involve other currencies, including American dollars, Korean won and Chinese yuan. Increasingly, it seems, intermediaries are being used to move the money, according to Eladio Barrado, a spokesman for Siat, the main union representing employees of Spain’s national tax agency.

In a prominent case last year, named Operation Emperor, the authorities broke up a ring that was using a fake import-export company to launder hundreds of millions of euros belonging to Chinese criminal gangs and wealthy Spaniards. The network was smuggling cash out of Spain by train and car.

That case resulted in the arrests of more than 100 people. But in most cases, people caught moving undeclared money are dealt with more discreetly, in private judicial proceedings, and are fined rather than imprisoned.

In the case of Boillon, the former diplomat, news of the cash seizure leaked out almost immediately to the French investigative website Mediapart. His case is pending before the National Judicial Customs Service. Efforts to reach him for comment or to leave messages at the registered Paris address of his business were unsuccessful.

–Doreen Carvajal and Raphael Minder / New York Times