LONDON — Some of the text messages read like real-time rallying calls for rioters.

"If you're down for making money, we're about to go hard in east London," one looter messaged before the violence spread.

Others directed troublemakers to areas of untapped riches — stores selling expensive stereo equipment, designer clothes, alcohol or bicycles.

Encrypted messages sent via BlackBerrys were being used by mobs to encourage rioting that has spread from London across central and northern England for a fourth night of violence driven by poor, diverse and brazen crowds of youths.

Scenes of ransacked stores, torched cars and blackened buildings frightened and outraged Britons just a year before London is to host the summer Olympic Games, and brought demands for a tougher response from law enforcement.

London's Metropolitan Police department put thousands more officers in the streets and said that by Wednesday there would be 16,000 — almost triple the number present Monday.

Although London saw no new unrest late Tuesday, a police station in the central England city of Nottingham was firebombed by a 40-person-strong mob, and hundreds of youths battled police in the northwestern city of Manchester.

Some London residents prepared to defend their homes and stores. Stephen Lennon, leader of the far-right English Defense League, said 1,000 members planned to turn out in Luton, where the group is based, and other areas that have suffered unrest, including Manchester, in an attempt to quell the violence.

The far-right group was cited as an inspiration to Anders Behring Breivik, who has confessed to the July 22 massacre in Norway. Outside a Sikh temple in Southall, west London, residents stood guard and vowed to defend their place of worship if mobs appeared. Another group marched through Enfield, in north London, aiming to deter looters.

Police shooting

Britain's riots began after last week's police shooting of Mark Duggan, a 29-year-old father of four. According to British media, one of the last messages Duggan sent was via BlackBerry's messaging system.

"The Feds are following me," he allegedly wrote to his girlfriend, according to The Daily Telegraph.

An initially peaceful protest over the shooting in London's Tottenham neighborhood turned violent. That clash has morphed into a general lawlessness that police have struggled to halt with ordinary tactics.

Since Monday, 685 people have been arrested in London and 111 people have been charged, including at least 69 with burglary, six with handling stolen goods, two with assault on police and five with possession of offensive weapons.

In Manchester, police said they have made at least 47 arrests, including one man on suspicion of using Facebook's social-networking site to incite disorder.

In Birmingham, West Bromwich and Wolverhampton, police have arrested 229 people since Saturday night for offenses including violent disorder, burglaries, criminal damage and looting.

While the rioters have run off with sneakers, bikes, electronics and leather goods, they also have torched stores apparently just for the fun of seeing something burn.

Many of the masked or hooded youths have been photographed typing cellphone messages while cars and buildings burn.

BlackBerry's messaging system is popular among youths because it's free, compatible with multimedia and private, compared with Facebook and Twitter. Its encrypted messages give troublemakers an added benefit: Police aren't able to immediately trace message traffic the way they can with regular cellphones.

BlackBerry said it was cooperating with police, but shutting down the messaging system could penalize more than just the troublemakers. More than 45 million people use the BlackBerry messaging system worldwide. President Obama is said to use the same secure system to communicate.

BlackBerry maker Research in Motion, based in Waterloo, Ontario, posted a message on its official U.K. Twitter account last night saying, "We feel for those impacted by the riots in London. We have engaged with the authorities to assist in any way we can."

On Tuesday, hackers compromised BlackBerry's blog site in response to the company saying it would cooperate with police.

Social media have been used to coordinate demonstrations in the Mideast, to campaign for Saudi women's right to drive and to call for lower prices for cottage cheese in Israel.

In a Twitter post, David Lammy, a member of Parliament from Tottenham and a former intellectual-property minister, called for a suspension of BlackBerry's encrypted instant-message service.

Contrasts

Britain is full of contrasts between the haves and have-nots, where areas of soot-stained apartment buildings are a stone's throw from Buckingham Palace. It is also a place where the class system is imprinted on the country's social fabric, seen clearly in the political and business elite.

"You have groups who are highly technically integrated but socially completely outclassed and alienated," said Rodney Barker, emeritus professor of government at the London School of Economics.

The past year has seen mass protests against the tripling of tuition fees and cuts to public-sector pensions. In November, December and March, small groups broke away from large marches in London to loot.

According to July figures from Britain's Office of National Statistics, one in five 16- to 24-year-olds is unemployed — the highest rate of youth unemployment in some 20 years. Overall unemployment rates, however, have remained stable.

Compiled from The Associated Press, Bloomberg News and The New York Times