It follows two nights of violence over the weekend which started after police shot a man dead in Tottenham.
London's mayor Boris Johnson is cutting short his holiday to return to the city, where more than 200 people have been arrested and 35 officers injured.
Home Secretary Theresa May also returned early from holiday, to meet Acting Metropolitan Police (Met) Commissioner Tim Godwin and other senior officers.
Deputy Assistant Commissioner Stephen Kavanagh said they discussed the police response to the violence.
He said: "They were looking at preparations for tonight (Monday) to make sure any violence is dealt with quickly."
She said: "The riots in Tottenham on Saturday night and the subsequent disturbances in other parts of London are totally unacceptable."
She also paid tribute to the bravery of police officers and urged local communities "to work constructively with the police to help them bring these criminals to justice".
Mrs May said at least 215 people had been arrested and 25 people charged.
A peaceful protest in Tottenham on Saturday over the fatal shooting by police of Mark Duggan, 29, was followed by violence which spread into Sunday.
A candlelit vigil was due to be held at The High Cross in Tottenham on Monday evening.
A BBC journalist said the latest violence started in Mare Street, Hackney, when a man was stopped and searched by police but nothing was found.
Riot vans were drafted in and there are up to 200 police officers in riot gear in the Hackney area.
Groups of people began attacking the police at about 16:20 BST, throwing rocks and a bin at officers.
At the sceneIn a lane off Mare Street the wreckage of a burnt-out car still smoulders, surrounded by riot police.
At least for now, the violence in this half of London seems to have abated. I was talking to one young man who had received on his BlackBerry a list of places where he said there will be further trouble tonight.
He didn't tell me which places and stressed it is speculation. But he and a friend told me frustration with poverty in the area was boiling over.
On Mare Street there is the sound of crunching as police vans run over broken glass. Much of it from a bottle bank which was overturned providing makeshift missiles for rioters who lobbed the bottles at police.
Planks of wood taken from a lorry were then hurled towards lines of riot officers.
Violence has also broken out in Peckham, south London, where a shop and a bus have been set on fire.
In nearby Lewisham, footage showed a number of vehicles and a trail of bins on fire.
Deputy Assistant Commissioner Stephen Kavanagh said there were "significant resources" on the streets, with a third more officers on duty than on Sunday.
He said: "What we can see is that the Metropolitan Police are getting police officers there in numbers.
"When we have large numbers of criminals intent on that type of violence, we can only do that, get lots of officers there quickly and try to protect local businesses and local people."
He also admitted relations with the family of the man shot dead by police could have been handled better.
Mr Kavanagh said: "I want to apologise to the Duggan family because I think both the IPCC (Independent Police Complaints Commission) and the Metropolitan Police could have managed that family's needs more effectively."