In what appeared to be coordinated raids, the FBI executed 40 search warrants against alleged members of hacking collective Anonymous hours after Britain arrested five people over service attacks last year.
Anonymous denounced the British arrests as a "serious declaration of war".
The FBI did not reveal if arrests resulted from its raids.
ITnews believed the raids were linked to attacks on the sites of payment companies Mastercard and Visa that severed payment system support to WikiLeaks late last year.
The FBI said it was working with European authorities to stop the Anonymous denial-of-service campaign.
"Facilitating or conducting a [distributed denial of service] attack is illegal, punishable by up to 10 years in prison, as well as exposing participants to significant civil liability," the agency said.
Those arrested in Britain faced similar, lengthy prison sentences.
In an open letter, Anonymous denounced the sentence threats and the arrests for what it said amounted to "peaceful protests" that caused no lasting damage to the websites it targeted.
The loose-knit collective said the arrests were politically motivated because there were few attempts to track those responsible for allegedly denying service to WikiLeaks when it started leaking US embassy cables.
"The fact that thousands of people from all over the world felt the need to participate in these attacks on organisations targeting WikiLeaks and treating it as a public threat, rather than a common good, should be something that sets you thinking," Anonymous said.
"You can easily arrest individuals, but you cannot arrest an ideology.
"We will not rest until our fellow anon protesters have been released."