British police working with the FBI arrested a 19-year-old man over attacks by a hacker group on businesses and government agencies including the CIA, US Senate and Sony, Scotland Yard said.
The man, named in reports as Ryan Cleary, was detained at a house in the suburban town of Wickford in Essex, southeast England, in connection with a month-long global rampage by the Lulz Security group.
He was being questioned on Tuesday, local time, at a central London police station.
"Yes, the arrest is in connection with the Lulz Security attacks. We believe this to be a significant arrest," a police source confirmed on condition of anonymity.
But the group played down the arrest in a posting on its official Twitter account.
It has announced previous hacking attacks on the same account that turned out to be genuine.
"Seems the glorious leader of LulzSec got arrested, it's all over now... wait... we're all still here! Which poor bastard did they take down," said the posting.
In a statement, Scotland Yard said its specialist cyber-crime team had arrested the man in a "pre-planned intelligence-led operation" on suspicion of computer misuse and fraud offences.
"The arrest follows an investigation into network intrusions and Distributed Denial of Service (DDoS) attacks against a number of international businesses and intelligence agencies by what is believed to be the same hacking group," it said.
DDoS attacks overwhelm websites with traffic, making them sluggish or unresponsive.
"Searches at a residential address in Wickford, Essex, following the arrest last night have led to the examination of a significant amount of material. These forensic examinations remain ongoing," the statement said.
British police had been "working in cooperation with the FBI" in the run-up to the arrest, it added.
Cleary's mother told the BBC that her son had been arrested and said that since he was 12 years old "computers were his world."
The latest in a series of hacking groups to gain public prominence, Lulz knocked out the CIA's public website, cia.gov, for about two hours last week using a DDoS attack and also hacked into the US Senate's public website.
The group has also released tens of thousands of user names and passwords stolen from Sony and other sites, and on Monday Lulz targeted the website of Britain's Serious Organised Crime Agency.
British authorities said Tuesday they were investigating whether information from the country's 2011 national census had been hacked.
"We are aware of the suggestion that census data has been accessed. We are working with our security advisers and contractors to establish whether there is any substance to this," the Office of National Statistics said in a statement.
But Lulz later said on Twitter that it was not responsible, adding that it had itself been the victim of a hoax statement.
In an online manifesto posted last week, Lulz - whose name is a derivative of the text shorthand for LOL, or "laugh out loud" - said they were staging the attacks for their own entertainment.
"You find it funny to watch havoc unfold, and we find it funny to cause it," it said.
"For the past month and a bit, we've been causing mayhem and chaos throughout the internet, attacking several targets including PBS, Sony, Fox, porn websites, FBI, CIA, the US government, Sony some more, online gaming servers," Lulz said.
Lulz last week denied reports that it was in conflict with the hacker group Anonymous, which gained notoriety last year with cyberattacks in support of controversial whistleblowing website WikiLeaks.
Anonymous in May posted Ryan Cleary's personal details on the internet after accusing him of trying to hack into its chatrooms.