Rioters have set fire to a Miss Selfridge shop on Market Street in Manchester city centre.
Greater Manchester Police said it had been "engaged" with dealing with outbreaks of minor disorder in Salford and Manchester city centre this afternoon, involving "a small number of youths".
Assistant Chief Constable Terry Sweeney said: "A handful of shops have been attacked by groups of youths who have congregated and seem intent on committing disorder.
"As we have said, we will not allow such mindless criminal damage and wanton violence to go unpunished and we will arrest and prosecute anyone found to be involved in looting or acts of criminal damage.
"Increased numbers of officers are on the ground both responding to any incidents and dispersing any groups of youths before they commit any crimes.
"We have already made a number of arrests and will continue to target those who seem determined to commit disorder.
"I want to take this opportunity to appeal for calm among members of our community who maybe concerned. We would advise they stay out of the city centre while we assess the scale of any disorder and report any concerns they have to us."
Mr Sweeney added: "Lastly, I again want to remind people to follow GMP's official responses on our social networking sites and not believe some of the rumours and speculation.
"Such gossip is unhelpful and causing unnecessary fear in our communities. We have already made one arrest of a man on suspicion of using Facebook to incite disorder and we will not hesitate to arrest anyone involved in this type of criminal activity."
Earlier, hundreds of youths were involved in a stand-off with riot police four miles away at Salford Shopping City.
A Bargain Booze was looted and the windows of the Money Shop smashed.
Smoke also billowed from a council housing office, apparently set on fire during the trouble.
Mr Sweeney posted on the force's Twitter account: "Aware of pockets of minor disorder in Salford city centre. Advising people to stay out of city centre but also keep calm."
Around 100 youths looted Foot Asylum in the Arndale Centre after two raiders smashed open the glass entrance with a large stone slab.
Once the glass was shattered, youngsters rushed in and carried out clothing and shoes.
They dispersed towards Deansgate when a police van arrived at the scene and a lone officer in riot gear stepped out.
Officers with dogs then began to patrol the area.
A line of officers then blocked the bottom of Market Street at the junction with Cross Street.
Sporadic looting broke out across the city centre, with gangs playing cat and mouse with riot police. Looters broke into a Diesel clothing shop and a Bang & Olufsen store.
They threw stones and other missiles at shop windows, whooping and shouting as they ran from police.
A cafe on Deansgate also had its windows smashed.
Riot police in vans chased large groups of youths wearing ski masks and hoods as they rampaged through the city streets.
Cheers went up among the crowd as the front window of the Bang & Olufsen store, off King Street, was smashed in.
Some wore hoods up and scarves to mask their faces but others brazenly walked in.
One pair carried out a widescreen TV while others gleefully pocketed electrical items.
There was some degree of co-ordination, with several of the gang shouting out directions for the others to follow.
One again the mob scattered along different routes as police sirens sounded.
Five minutes later, several hundred youths marched down Princess Street towards John Dalton Street.
Those at the front were brandishing sticks.
One spotted a reporter and mistook him for a police officer. A bottle of spirits was launched in his direction but narrowly missed.
Among the marchers were children on pedal cycles.
The Tesco Express store on Princess Street was next to fall victim to the looters.
A hardcore of around 200 seemed hellbent on looting while a similar number were caught up in the novelty of the events as they encouraged the disorder.
Other stores looted were the Life designer clothes shop in Old Bank Street and mobile phone stores 3 and Orange in St Ann's Square.
Other shops in the square remained untouched.
All shops in the city centre had closed early but many pubs and restaurants remained open.
The looting appeared to be dissipating as police vans were stationed at damage sites.
Large groups of people began to gather along Deansgate.
Looters helped themselves to bottles of alcohol from Sainsbury's Local at the corner of Bridge Street.
The thieving continued for several minutes in front of onlookers.
All had grabbed what they wanted and disappeared into the side streets before three police vans arrived.
Despite riot police positioned across the city, gangs of youths on mountain bikes, their faces masked, prowled the streets.
On occasions they could be seen talking to drivers of cars on mobile phones, exchanging information, while they drove around the streets in what appeared to be co-ordinated manoeuvres.
A jewellers was also reportedly attacked before plain-clothed police nearby ran in to arrest two looters from the shop.
Stand-offs with police continued around Salford shopping precinct.
A spokeswoman for Greater Manchester Police said up to 200 youths were involved at about 7pm.
According to reports, the confrontations were continuing as officers battled to corral them.
Isolated looting continued in Manchester city centre.
Many small groups of male youths could be seen wandering around the centre. Two individuals jumped into a doorway as a police tactical aid unit drove past.
Glen Barkworth, general manager of the Arndale Centre, described how the rioting began in Cathedral Street at 5.30pm tonight.
He told Sky News: "I came into the centre and I witnessed what was going on from an upper level, and to see the public having to flee in terror as the youths were bounding through the centre (of the city) was a sight I hope I never have to see ever again."
Mr Barkworth said the Arndale Centre itself was breached on one occasion by around 100 youths, but no members of the public or staff were in the area affected at the time.
In Piccadilly Gardens an amusement arcade had been plundered.
Machines were overturned in Piccadilly Museums with coins strewn across the carpet.
H Pollock Ltd Wholesale Jewellers Ltd in Withy Grove, Shudehill, had also been raided with part of its metal shutter mangled.
Four glass repair vans were parked up at Printworks.
Officers have now cordoned off the surrounding areas around the Arndale Centre.
Mounted officers were now guarding Exchange Square, while they also formed a ring of protection around Market Street.
St Anns Square was also now blocked off as was a section of Cross Street.
Deansgate was still open to traffic but largely deserted as the vast majority of troublemakers appeared to have pushed up to the Northern Quarter and around Piccadilly Gardens.
There were smaller pockets of youths who menacingly roamed the backstreets.