The Met's deputy commissioner Paul Stephenson made the following statement at 09.50 outside New Scotland Yard, the police headquarters 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."

POLICE UPDATE AT 11.50am:
'Plot has global dimensions'

The Met's anti-terror chief, deputy assistant commissioner Peter Clarke, said the following at Central Hall in Westminster: "I would like to give you some details about the counter terrorist operation, which took place overnight. I shall focus purely on the criminal investigation. As always, I must preface my remarks with a request for restraint - and by that I mean restraint in the reporting of events so far as they relate to those who have been arrested. We must all, please, be careful to say or do nothing that might prejudice the right of any individual to a fair trial.

"The investigation has focused on intelligence, which suggested that a plot was in existence to blow up transatlantic passenger aircraft, in flight. The intelligence suggested that this was to be achieved by means of concealed explosive devices smuggled onto the aircraft in hand baggage. The intelligence suggested that the devices were to be constructed in the United Kingdom, and taken through British airports. The number, destination and timing of the flights that might be attacked remain the subject of investigation.

"This has been, and continues to be, a fast moving investigation.

"Today marks the culmination of one phase of what is a major operation that has already lasted several months, and will undoubtedly last long into the future.

"During the investigation an unprecedented level of surveillance has been undertaken and our objective is to gather intelligence and evidence in support of the investigation. We have been looking at meetings, movements, travel, spending and the aspirations of a large group of people. This has involved close co-operation, not only between agencies and police forces in the United Kingdom, but also internationally.

"As is so often the case in these investigations, the alleged plot has global dimensions.

"The investigation reached a critical point last night when the decision was made to take urgent action in order to disrupt what we believe was being planned. As always in these cases, the safety of the public was our overriding concern.

"Throughout the night a significant number of arrests were made. All the arrests were made on suspicion of the commission, preparation or instigation of Acts of terrorism under the Terrorism Act 2000.

"The people arrested remain in custody in London where officers will pursue the investigation from the Metropolitan Police Anti-Terrorist branch.

"Searches are also underway at a number of business and residential premises in London and elsewhere. We anticipate that these will take some time to complete.

"You will realise that this is a very early stage in what will be a meticulous, painstaking criminal investigation. We will now be embarking on the next stage of the investigation, an investigation that will take us wherever the evidence leads. As and when I am able to do so, I will give out information on the progress of the investigation. But please be aware that we will only release specific information when we are sure it is correct.

"I am sure you will understand that at this very early stage it is not appropriate for me to answer questions. I will give you further information only when I am able and it is appropriate to do so. At this point I must focus on the criminal investigation and any subsequent legal process, and refrain from speculation."