The Met's deputy commissioner Paul Stephenson made the following statement at 09.50 outside New Scotland Yard in London:
"We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and commit mass murder.
"The Commissioner has been fully briefed and he is at NSY today. He and I want to pay tribute to officers from the Met's Anti-Terrorist Branch and the security services for the work they have undertaken to disrupt these activities.
"We believe that the terrorists' aim was to smuggle explosives onto airplanes in hand luggage and to detonate these in flight.
"We also believe that the intended targets were flights from the UK to the USA.
"Overnight we have arrested 21people. The majority of those arrests have been in London but we have also made arrests in Thames Valley and Birmingham. We are currently searching a number of addresses. We are grateful to our colleagues in other force areas for their support. DAC Peter Clarke Head of the Anti-Terrorist Branch will give further details about this part of the operation later this morning.
"As the operation got underway we spoke to community leaders to make them aware that a major anti-terrorist operation was taking place but without giving specific details about locations. This extensive dialogue will continue throughout the operation.
"We would like to reassure the public that this operation was carried out with public safety upper most in our minds. This is a major operation, which will inevitably be lengthy and complex. The UK is now at the highest possible level of alert. We will consider the threat in its wider sense and take whatever action is necessary to protect people in London and throughout the Country.
"Measures have already been put in place to restrict hand luggage taken onto aircraft. This will inevitably cause significant delay and we ask people to remain calm, patient and vigilant at this very difficult time.
"Put simply, this was a plot to commit mass murder on an unimaginable scale."