A network of online fraudsters who masqueraded as European crime-fighting agency Europol and collected millions of euros in fake fines has been broken up - by Europol.

The group, which worked across 30 countries over the past two years, paralysed computers with a virus and left messages purporting to be from organisations like Europol and the police, saying users could only regain access to their machines if they paid a fine.

"It's impossible to know for sure how many citizens were affected by this, but we estimate hundreds of thousands of Europeans were," Europol Director Rob Wainwright said at a news conference in Madrid on Wednesday.

"If we take into account that the average fine was 100 euros ($130) and 3 percent...paid it, then the estimated damage is millions of euros," he said. Wainwright added his own name had been used to trick Internet users.

The virus was known as "Ransomware" and had up to 48 different mutations to overcome anti-virus software.

The leader of the fraud network, a 27-year-old Russian citizen, was arrested in December in the United Arab Emirates, Spain's secretary of state for security, Francisco Martinez, said at the news conference.

Spanish police arrested 10 members of the group last week on the country's southern Costa del Sol, a popular tourist destination, Martinez said.

Six of the detainees were Russians, two were Ukrainian and two from Georgia. These members mostly took care of money laundering and sending cash electronically to Russia, while the head of the group was responsible for developing the virus.