London (CNN) -- Heavy policing has brought calm to London after several days of rioting and looting, but trouble has continued to spread to other cities around the country.
Where did the rioting begin?
In Tottenham, north London, an ethnically diverse area where locals had been protesting Saturday about the death of 29-year-old Mark Duggan, a black man who was shot in a police operation on Thursday.
Where has the rioting taken place?
In several areas of London and other major British cities, such as Birmingham and Gloucester in central England, Manchester, Salford, Liverpool and Nottingham further north and Bristol in the southwest.
After the initial outbreak in Tottenham overnight on Saturday, violence also spread on Sunday the north London suburb of Enfield, Brixton in the south and Oxford street in central London, the capital's main shopping district.
On Monday afternoon large gangs roamed Peckham and Hackney in east London, looting shops, attacking buses and setting cars and shops alight.
Later on Monday trouble spread to the leafy London suburb of Croydon, where several buildings, mainly shops, were set on fire. In Enfield, a large Sony distribution center was torched.
On Monday evening rioting erupted in affluent Battersea in southwest London, where -- in what has now become a familiar pattern -- gangs of masked youths looted shops. Many other trouble flashpoints were reported across the city.
On the same night, there were reports of violence in Liverpool, Birmingham and Bristol.
Tuesday night saw an uneasy calm restored in London after the police presence was more than doubled. But rioting continued in other major cities.
Why did the riots spread?
Police say the subsequent riots in other parts of London and the UK are copycat events conducted by opportunists and criminals.
What options do the police have to quell the unrest?
The police introduced special powers in four areas of London on Sunday -- Lambeth, Haringey, Enfield and Waltham Forest -- allowing stop and search without reasonable suspicion in a bid to keep rioters off the streets.
In theory, parliament could invoke powers to implement curfews, use water canons or even call in the armed forces.
However, senior politicians and police officers have said that these options are unlikely to be used unless the situation got significantly worse.
In 1981 police used CS gas for the first time to control civil unrest in mainland Britain during the Toxteth riots in Liverpool, northwest England.
Are there enough police officers?
A video report from Battersea in south London on Monday evening showed gangs of youths roaming the streets, smashing and looting shops with apparent impunity. The Sky News reporter who shot the scenes on his mobile phone said he could not see a single policeman. There were similar reports from other parts of London leading to claims that there were not enough police to keep the peace.
The deployment of 16,000 police officers on Tuesday night appeared to work, but if this needed to be sustained over a longer period, it could create problems.
Several hundred additional officers have been drafted into London from neighboring counties including Kent, Essex, Surrey, Northamptonshire, Sussex and Thames Valley Police, whose officers patrolled London's central shopping district on Monday evening.
The head of London's Metropolitan Police has called for all Specials -- volunteer police officers -- to report for duty, as the capital's law enforcement resources are stretched thin. It was also announced that all police leave has been cancelled.
How did Mark Duggan die?
Officers from Operation Trident -- the Metropolitan Police unit that deals with gun crime in London's black communities -- stopped the cab Duggan was travelling in during a pre-planned operation.
Duggan, a father of four, died of a single gunshot wound to the chest, an inquest at north London Coroner's Court heard on Tuesday.
There was a suggestion that officers could have come under fire from the car carrying Duggan.
However, the Independent Police Complaints Commission said Tuesday that there is no evidence Duggan opened fire at officers.
Why did the police want to arrest him?
Trident officers, backed up by a CO19 detachment -- the Metropolitan Police's specialist gun unit -- were conducting a "pre-planned" operation to arrest Duggan.
There have been conflicting reports about Duggan's background. Some have painted him as a suspected gang member, others as a loving family man unlikely to become involved in violence.
Police say he was carrying a loaded firearm at the time of his shooting.
Was social media a factor in spreading the violence?
London's Deputy Assistant Commissioner Steve Kavanagh said that smart phones and social networks like Twitter had been used by criminals to liaise. "It's a group of individuals using modern technology to cause chaos," he said.
Other reports say BlackBerry's messaging service was a popular means of communication to spread news about the violence.
Are there any parallels with previous riots in north London?
Only superficially, but some might argue that today, as in 1985 when riots erupted in Tottenham, there is a racial element to the troubles.
In 1985 Floyd Jarrett, who was of Afro-Caribbean origin, was stopped by police near the Broadwater Farm estate in Tottenham on suspicion of driving with a forged tax disc, a legal requirement for almost all cars in the UK.
A few hours later officers raided the nearby home of his mother, who collapsed and died during the raid. Like the recent troubles, it was a protest outside Tottenham Police Station which sparked the 1985 conflict.
Police officer Keith Blakelock was stabbed to death by a gang during the riots as he tried to protect fire crews.
Is there a connection with the riots in the UK last year?
No. They were student riots against tuition fee rises, although they were also marred by a certain amount of violence. But there was no looting and the protests were organized and publicized in advance. The protests were also mainly concentrated in London as students wanted to demonstrate against politicians.
The IPCC is conducting an investigation into Duggan's death. Colin Sparrow, deputy senior investigator for the IPCC, told an inquest into the death on Tuesday that their "complex investigation" could take four to six months.
The IPCC commissioner said in a written statement: "[We are] investigating not only the actions of the officer firing the shots but also the planning, decision making and implementation of the police operation. Our lines of enquiry include the bullets fired and any firearms used and recovered."