All phones at its Scotland Yard headquarters were cut off and the central 999 call system also failed, leaving calls to be switched to local police stations.
The IT systems logging emergency calls failed, as did a backup system at Hendon.
The debacle was caused by a workman cutting through an electricity cable in the Victoria area.
Backup systems restored power to the building but officers were left using their mobiles to carry on work.
London's 999 ambulance service was also hit, with controllers using pen and paper while power was disrupted.
Graham Titterington, an analyst with Ovum, said, "It should not be possible to bring system down with any one single point of failure - whether that is in power supply, hardware and cabling or software."
Harry Fletcher, deputy general secretary of the probation officers' union, said, "This failure is very serious. Not only are the police unable to access their systems, there is a danger that vital information on criminals and potential terrorists has been lost."
ComputerWeekly.com could not get through to Scotland Yard for comment.