Deputy Commissioner Paul Stephenson of Scotland Yard today said a plan by terrorists to commit "mass murder" in the UK had been disrupted.
He said: "We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit, quite frankly, mass murder.
"I would want to join the commissioner who is fully briefed and has been here at New Scotland Yard since very early this morning in paying tribute to the Met's counter terrorist branch and the Security Services for the work that they have undertaken in disrupting these activities.
"We believe that the terrorists' aim was to smuggle explosives onto aeroplanes in hand luggage and to detonate these in flight.
"We also believe that the intended targets were flights from the United Kingdom to the United States of America.
"I can confirm that a significant number of people are currently in custody and the operation is ongoing.
"The majority of those arrests have been here in London, but we have also made arrests in Thames Valley and in Birmingham and of course I am very grateful for the support our colleagues have shown in backing up this operation, and showing their leadership in those forces.
"We are currently searching a number of addresses and Deputy Assistant Commissioner Peter Clarke, head of the anti-terrorist branch - who is known to many of you - will give further details about the investigation later this morning.
"As the operation got under way we spoke to a good number of community leaders to make them aware that a major anti-terrorist operation was under way but without giving specific details about locations.
"This extensive dialogue will continue.
"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 United Kingdom 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 here in London and right the way throughout the United Kingdom.
"Measures have already been put in place to restrict hand luggage taken onto aircraft. This will inevitably cause very significant delays and I know it is doing that as we speak here. We ask for people's continued help and patience at these very difficult times.
"We are genuinely looking to the public to remain calm, patient and vigilant
"But we cannot stress too highly the severity that this plot represented.
"Put simply this was intended to be mass murder on an unimaginable scale."
In a question and answer session immediately after the statement he said that 21 people were currently in custody.
He added: "The operation is ongoing, we have been very successful in arresting those we were targeting, this is a lengthy investigation and no doubt there will be further developments."
He added: "This is an operation that's been going on for some considerable time and we moved in because we thought it was the right thing to do in terms of reducing and minimising crime to the public.
He would not discuss how many planes police believe were being targeted in the alleged plot, only saying "that is something the operation will continue to investigate."
He concluded: "This is not about communities, this is about criminals, murderers, people who want to commit mass murder, this is not about anything to do with any particular community.
"This is about people who might masquerade within the community behind certain faiths... this is about people - desperate, desperate people - who want to do things that no right minded citizen of this country, or any other country, would want to tolerate."