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Police described the violence across London since Saturday night as “copycat criminality.” They said more police will be put on the streets to restore order.

“This has changed from a local issue into organized criminality,” police Deputy Assistant Commissioner Steve Kavanagh said Monday as he announced a “momentous investigation” to track down the perpetrators.

The perpetrators were relatively small groups of youths, their heads and faces covered, who used social media such as Twitter, cellphone text messages and BlackBerry instant messaging to organize and keep a step ahead of police.

One BlackBerry message Sunday reportedly urged young people from across London to converge on Oxford Circus for “pure terror and havoc & free stuff.”

Commissioner Kavanagh told reporters his officers would “absolutely” consider arresting tweeters for incitement if they had urged or orchestrated violence.

But most Twitter users appeared critical of the riots or, at least, embarrassed by events.

One wrote: “The Youth of the Middle East rise up for basic freedoms. The Youth of London rise up for a HD ready 42” Plasma TV.”

c Louise Osborne in London and Shaun Waterman in Washington contributed to this report.