UPDATED 7.48pm: A major terrorist plot to blow up multiple aircraft in mid flight from Britain to America has been thwarted and 24 people arrested, police have said.

"Put simply, this was intended to be mass murder on an unimaginable scale", the Met's deputy commissioner Paul Stephenson said in a strongly-worded statement this morning.

"We believe that the terrorists' aim was to smuggle explosives onto airplanes in hand luggage and to detonate these in flight."

A total of 24 people were arrested overnight - mostly in London, but also in the Thames Valley and Birmingham, police have confirmed. The suspects, thought to be British citizens, are being questioned.

Both Britain and America are on their highest level of alert.

Home secretary John Reid said the plot was believed to be "a very significant one", bigger than any other the UK has seen.

The planned explosions may have been simultaneous on as many as 10 aircraft, possibly over the Atlantic as to minimise forensic evidence, according to reports.

It involved hiding masked liquid explosives in carry-on luggage. All liquids are being removed from anyone travelling out of UK airports today.

The scheme is "suggestive of an al-Qaida plot", the Bush administration said in Washington.

Airport chaos

For the first time ever all UK airports, including the two busiest at Heathrow and Gatwick, have been put on "critical" security alert at 2am this morning. "Critical" alert, the highest warning level, is defined by MI5 as fear of an "imminent" terrorist attack.

Over 400,000 passengers have been affected by severe delays and flight cancellations countrywide.

Heathrow has been closed to all incoming aircraft not already in the air, while all outbound short-haul flights were grounded until 3pm.

Brussels international airport, the German airline Lufthansa, Air Iberia and the Greece national carrier airline Olympic Airlines have cancelled all flights to Britain today.

No hand luggage is being allowed on UK planes, apart from a few exceptions like wallets, keys and passports. These can be carried in a transparent plastic bag.

All other items, including laptops, mobile phones, iPods, books and newspapers, have to be searched and checked in along with normal luggage. Baby milk can be carried on board, but must be tasted in front of airport staff.

The unprecedented safety measures have already led to substantial delays. Passengers are waiting more than five hours to go through security, with queues snaking outside terminals.

In America, the threat level has also been raised to the maximum of "red" for commercial flights from Britain. No liquids, including beverages, lotions and hair gels, are allowed to be carried onto any flights, whether domestic or international. It is the first red alert in the US triggered by a threat in another country.

The American secretary of homeland security, Michael Chertoff, said the terror scheme "was sophisticated, it had a lot of members and it was international in scope. It was in some respects suggestive of an al-Qaida plot."

FBI Director Robert Mueller agreed: "This had the earmarks of an al-Qaida plot."

Chertoff said the plotters were "really getting quite close to the execution phase".

Back in London, the Met's Mr Stephenson said: "We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and commit mass murder."

Two US security officials have been quoted saying the targeted airlines were United, American and Continental.

Attacks 'not today'

This morning's arrests were part of a "pre-planned intelligence led operation" by anti-terrorist police and MI5, the Met said in a statement.

It is not thought the attacks would have gone ahead today, but police chose it at their moment to swoop after several months of surveillance.

Searches are still ongoing at several addresses, including five homes in Walthamstow, east London, and another four in High Wycombe, Buckinghamshire.

"This is a major operation, which will inevitably be lengthy and complex," Mr Stephenson added.

Police chiefs and deputy prime minister John Prescott have briefed community leaders about the investigation. Mindful of the outrage after the botched raid on a house in Forest Gate, east London, Mr Stephenson stressed Muslims are not being targeted.

"This is not about communities: it is about criminals, murderers, people who want to commit mass murder. This is about people who might masquerade in the community, hiding behind certain faiths, but who want to commit acts that no right-minder person would want to applaud."

Passengers were advised to check with their airlines before travelling to airports. People due to travel from Heathrow, Gatwick and Stansted should only fly if "absolutely necessary", the airports authority BAA has said.