Anti-terrorist officers believe the same IRA team was responsible for yesterday morning's attacks and the devices detonated on the Finchley Road, also in north London, late on Friday night.
Commander David Tucker, head of the Anti-Terrorist Branch, said no warning had been issued: 'It's a change of tactic and. . . it does indicate a sinister change of strategy.'
The five bombs went off at intervals just after 6.30am. Two exploded near a YMCA in Crouch End, two more about a mile to the west on the main A1 into London at Archway, near Highgate tube station, and the fifth a few hundred yards away near an art gallery in Highgate High Street. A sixth was found nearby outside an estate agent's and was defused.
Although some businesses were seriously damaged and many windows shattered, no one was seriously injured. Four people, including a 12-year-old newspaper delivery boy and a 14 year-old-girl on her way to school, were treated for shock in local hospitals. Police were forced to close a number of main roads and traffic in the area was severely congested.
Mr Tucker praised the bravery of Noor Allana, the driver of a bus who slewed his vehicle across the road to stop traffic and evacuated his two passengers after the Highgate High Street explosion. Mr Allana said: 'If I had been half a minute earlier, it would have been right by my bus.'
The Crouch End explosion badly damaged the offices of the Hornsey Journal, a local weekly newspaper, and a furniture manufacturer. A resident at the YMCA, who gave his name only as Kato, said: 'There was a loud crack and it spewed the shop front across the street.'
Detectives believe that after a lull during the summer since the Bishopsgate bomb, the IRA is now resuming its campaign in London and may stay away from the heavily protected more obvious targets in central London.
Tomorrow, the City of London, which has already restricted vehicle access to the Square Mile, will unveil the lastest step in its strategy to combat IRA attacks. It is setting up a message pager system which will give subscribers warnings of suspect bombs to speed up evacuation.
Belfast police said one person was slightly injured after a small explosion at a tyre depot in the docks area.
Two terrorists staged a one- night bombing campaign in Leeds city centre, Newcastle Crown Court heard yesterday.
Eamonn O'Donnell, 39, a student, and Sean Cruickshank, 23, a lorry driver - both members of the outlawed Irish National Liberation Army - admitted causing more than pounds 50,000 of damage with incendiaries in June last year. They will be sentenced tomorrow.