(Elise Amendola/AP)

An international police operation recently shut down the world's largest for-hire service that allegedly slowed and disrupted millions of websites using malicious cyber tools, officials said Wednesday.

Law enforcement officials say that administrators of Webstresser.org were arrested Tuesday, in Britain, Croatia, Canada and Serbia. The service was taken offline and its hardware was seized in the United States, the Netherlands and Germany, officials said.

Many of Webstresser's customers and targets were American; the United States was among the five countries most affected by Webstresser's operations, Europol said.

Customers allegedly paid Webstresser to carry out cyberattacks on financial institutions, governments and law enforcement agencies using a method known as a Distributed Denial of Service, or DDoS, bombarding a site with a flood of requests to overwhelm it, according to Europol. Webstresser sold DDoS services for as little as 15 euros a month, or about $18, Europol said. This granted virtually any individual with some means the power to devastate online services on a whim.

Security experts say that by sharing resources online, professional hackers are making it much easier for inexperienced people without technological know-how to execute cyberattacks.

“It used to be that in order to launch a DDoS attack, one had to be pretty well versed in Internet technology,” Europol said in a statement Wednesday. “That is no longer the case.”

Officials considered Webstresser the largest marketplace for DDoS services in the world. The cyberattacker counted 136,000 registered users and had orchestrated 4 million attacks, the police agency said. The site was believed to have been used by a British suspect to attack banks in the United Kingdom last year, causing hundreds of thousands of dollars in damages, according to the BBC.

Dubbed Operation Power Off, the investigation was led by the Dutch police and Britain's National Crime Agency, with support from Europol and other law enforcement agencies.

Law enforcement officials said the rise of cyberattacks for hire will require a deepened level of international cooperation.

“Criminals are very good at collaborating, victimizing millions of users in a moment [from] anywhere in the world,” Steven Wilson, the head of Europol’s European Cybercrime Center, said in a statement Wednesday. “We need to collaborate ... with our international partners to turn the table on these criminals and shut down their malicious cyberattacks.”