Riots flared in English cities and towns as London waited anxiously to see if thousands of police deployed on its streets could head off the youths who had rampaged across the capital virtually unchecked for three nights.
In Salford, part of greater Manchester in northwest England, rioters threw bricks at police and set fire to buildings on Tuesday night.
Television pictures showed flames leaping from shops and cars in Salford and Manchester, and plumes of thick black smoke billowing across roads.
In central Manchester, police said a clothes shop was set alight. "I can confirm a shop is on fire and 200 youths that gathered in the city centre have been chased by riot police and dispersed. Seven arrests have been made so far," a spokesman for Greater Manchester Police said.
A senior police officer said the rioting and looting in Manchester was an act of "senseless violence and senseless criminality" and the worst violence in the city in 30 years.
Committed by people with "nothing to protest against", it has brought "shame on the streets" of the city, Assistant Chief Constable Garry Shewan of Greater Manchester Police said.
Further south in West Bromwich and Wolverhampton, cars were burned and stores raided.
A gang of up to 40 men firebombed a police station in the central English city of Nottingham but no injuries were reported, as riots spread across Britain, police said.
"Canning Circus Police Station (in central Nottingham) fire bombed by a group of 30-40 males. No reports of injuries at this stage," said Nottinghamshire Police in a message on their official Twitter feed.
A message later said the fire had been extinguished and eight people had been arrested.
In Liverpool, a Reuters reporter saw police with riot shields pushing back youths hurling bricks.
London was mostly quiet after a huge boost in police numbers. Commuters hurried home early, shops shut and many shopkeepers boarded their windows as the city prepared nervously for more of the violence that had erupted in its neighbourhoods. Police promised to nearly triple their deployment on the streets.
Community leaders said the violence in London, the worst for decades in the huge, multi-ethnic capital, was rooted in growing disparities in wealth and opportunity.
Gangs have ransacked stores, carting off clothes, shoes and electronic goods, torched cars, shops and homes -- causing tens of millions of pounds of damage -- and taunted the police.
Prime Minister David Cameron, who cut short a family holiday in Italy to deal with the crisis, told reporters: "This is criminality pure and simple and it has to be confronted and defeated."
Cameron said he will recall parliament from its summer recess to address the unrest.
"We've been discussing actions that we will be taking to help the police to deal with the disorder on the streets of London," Cameron said.
"We will do everything necessary to restore order to Britain's streets. Scenes of people attacking police officers and fire crews, this is criminality, pure and simple, this has to be confronted and defeated."
His comments came as reports surfaced that a 26-year-old man who was shot in a car in Croydon, south of London, had died in hospital, making him the first casualty during the violence.
At least 525 people have been arrested as a consequence of the riots which escalated on Monday night, two days after a police station was attacked in the north London area of Tottenham following protests over the fatal shooting of a 29-year-old man, Mark Duggan, by police.
Britain's Independent Police Complaints Commission (IPCC) said on Tuesday that it had carried out ballistic tests on a handgun found at the scene, thought to have been owned by Duggan, and found that the gun had not been discharged.
The UK's Forensic Science Service is to carry out further tests on the weapon to verify this.
The IPCC's investigation also verified that the bullet found lodged in a police officer's radio handset was a standard issue police bullet, and that it showed signs consistent with being fired from a police gun, casting further doubt on claims that Duggan had fired the shot.
Rubber bullets considered
Meanwhile, London's Metropolitan Police said it would deploy 16,000 officers on the streets on Tuesday night - double the number on the streets on Monday night - to address any potential unrest.
As calls mounted for stronger measures, Deputy Assistant Commissioner Stephen Kavanagh said police would consider using baton rounds, rubber or plastic bullets.
Charlie Angela reports on the escalating violence in the UK
"There are pretty tough questions confronting the politicians," said Tim Friend, Al Jazeera's correspondent in London. "It will be concerning police and politicians greatly. Whether what's happening outside London has anything to do with the shooting [of Mark Duggan], is doubtful at the moment."
The administrators of a Facebook page, created in memory of Mark Duggan, posted this message: "Please we ask for this violence to stop. This isn't about Mark, it never was. His family never wanted nor thought any of this would happen.
"You people are using this as an excuse. If you had any respect, you would stop this before they bring in the Army, the whole world is looking."
Government officials have branded the rioters criminals and said the violence would have no effect on preparations for the 2012 London Olympic Games - though television images of blazing buildings and rioting were likely to dent the capital's image.
'Strong police response'
Reacting to the looting and violence spreading across the country, Ed Miliband, leader of the opposition Labour party, posted on his Twitter account: "Shocked by scenes in parts of London and Birmingham. We need strong police response to restore calm and for communities to work together."
Some activists and opposition politicians suggested that government spending cuts had driven disadvantaged and unemployed youths to violence.
But Mike Freer, conservative MP for Finchley and Golders Green, said that was "complete nonsense".
"These riots are about vandalism. To the best of my knowledge, we talked about cuts but most of them have not bitten yet. If Labour politicians really believe that, let's have a list of cuts that they think justified people turning to violence."
Others including David Lammy and Diane Abbott, MPs for the affected areas of Tottenham and Hackney, rejected the idea that the government's austerity drive should be blamed for the riots.
"Cuts don't turn you into a thief. What we saw was people thieving for hours," Abbott said.
Chukka Umunna, another Labour MP, said the violence in London was "totally opportunistic and utterly unacceptable".
"I think we have got to be very careful about seeking to draw general conclusions from a series of events around London to make some kind of historical judgement about what is going on," he added.