Europol has been preparing for the euro for more than a year

THE HAGUE, Netherlands (CNN) -- Starting a currency from scratch has its advantages: Euro notes have more and better security features than any existing national currency.

But the introduction of the euro -- and the criminal activity it's expected to bring -- pose what officials at the European police agency Europol are calling their biggest challenge ever.

Europol has been working for the past year and a half to prepare for the euro, says Derek Porter of the agency's anti-counterfeiting team.

"We are not naive enough to suggest it doesn't still present an opportunity for organised crime. That's the reason we're here," Porter says.

The unveiling of more information about the security details of the euro notes and coins in late August 2001 meant a stepping-up in the activities of the Europol team.

"The volume of problems is that high, and maybe the frequency of problems, that we need a well-coordinated action in all European member states," says Europol Deputy Director Willy Bruggemann.

"It's the first time that many of the European member states will have to work together so closely."

That cooperation has begun as many Europeans swapped money hoarded in local currencies into dollars to avoid having to declare it to tax authorities during the changeover.

For most participating countries the changeover lasted from January 1 until March 1, 2002.

The French government wanted a quick conversion -- by February 17 -- and instructed tax authorities to turn a blind eye to what is effectively tax evasion.

There was no leniency in the Netherlands, though, where banks must report cash conversions involving more than 15,000 euros.

Experts say the sheer volume of transactions could overwhelm enforcement.