In a statement, the police said the man used the online alias Topiary and had been picked up during a raid on a residence in the Shetland Islands, the rugged archipelago off the northeastern coast of Scotland. The police said they were also questioning a 17-year-old but had not arrested him.
On Twitter, Topiary described himself as a “simple prankster turned swank garden hedge.” His missives were often facetious, suggesting the handiwork of someone who relished playful language.
Lulz Security, the offshoot of a larger and more amorphous hacker group called Anonymous, has said it was responsible for attacks on the sites of PBS, the Senate, the Arizona Department of Public Safety and a company associated with the F.B.I.
The most recent post on Topiary’s Twitter feed is dated July 21, two days after law enforcement authorities announced the arrests of more than a dozen people in the United States, Britain and the Netherlands, who were accused of participating in online attacks at the instigation of Anonymous. “You cannot arrest an idea,” Topiary wrote.
In the United States, 14 men and women, mostly in their 20s, were charged in connection with an online attack last December against PayPal, after the online payment company stopped taking donations for WikiLeaks.
On Wednesday, in response to those arrests, Anonymous called on supporters to cancel their PayPal accounts. Shares in PayPal’s parent company, eBay, dropped 3 percent, in line with declines in other tech stocks.
A PayPal spokesman, Anuj Nayar, denied that any significant number of PayPal users had canceled their accounts in response to the call for a boycott. “We haven’t seen any changes to our normal operations, including account opening and closing,” Mr. Nayar said.
The attack on PayPal’s site last December slowed down the company’s system, but to such a small extent that it would have been imperceptible to customers, he said. At no point, Mr. Nayar said, was the Web site shut down.