There was chaos at British airports on Thursday, with security services put on high alert after police said they had foiled a terrorist plot to blow up 10 passenger planes traveling from the UK to the US. The plot bore all the hallmarks of al-Qaida, according to American officials. Scotland Yard said a plan to commit "mass murder on an unimaginable scale" had been thwarted. It was alleged that terrorists planned to smuggle explosive devices, said to be in liquid form that cannot be detected by airport X-ray machines, in hand luggage to blow up the airplanes over the Atlantic. Police said they had been gathering intelligence over the last five days and had arrested 21 people in London, Birmingham and High Wycombe in Buckinghamshire in connection with the plot. On Thursday afternoon, police evacuated a number of houses and businesses in the High Wycombe area. "The police, acting with the security service MI5, [have] carried out a major counterterrorism operation overnight to disrupt an alleged plot to bring down a number of aircraft through mid-flight explosions," said British Home Secretary John Reid at a Thursday afternoon press conference. "Had this plot been carried out, the loss of life to innocent civilians would have been on an unprecedented scale. The police operation is complex and is ongoing; 21 people have been arrested. The police are confident that the major players in this plot are in custody." The arrests triggered worldwide security alerts and could herald a new age of terrorism in which attackers have access to explosives that are easy to carry and conceal. Security experts said the liquid explosives could have been mixed into a lethal concoction during the flight and caused devastation on the scale of 9/11. "We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit, quite frankly, mass murder," said Metropolitan Police Deputy Commissioner Paul Stephenson. "We believe that the 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 United States." British authorities would not say whom they believed was behind the plot, while US officials said it was "in some respects suggestive of an al-Qaida plot." While Stephenson did not want to reveal the identity of the suspects held in custody, he said: "I can confirm that a significant number of people are currently in custody and the operation is ongoing." The alert has caused severe disruption to air travel, with some airlines suspending flights. On Thursday, British Airways cancelled all domestic and European flights and Air France, Lufthansa, Iberia, KLM and Olympic have cancelled all flights to the UK. El Al said it was not cancelling flights out of the UK, and a spokesman for El Al in London said that its business was operating as normal. However, the afternoon flight from Heathrow Airport was subject to a three-hour delay. Thousands of passengers remained stranded at airports. Gatwick Airport cancelled all flights, while Heathrow was closed to all incoming flights and several outbound services were cancelled. Other airports across the UK have also been severely affected. The British Airports Authority told passengers to fly only if their journey was essential. Prime Minister Tony Blair briefed US President George W. Bush, and the US Department of Homeland Security increased the threat level to US-bound passenger flights originating in the UK to red - the highest threat level possible, and the first time it has done so. Blair's Downing Street office said there were no plans for him to cut short his Caribbean vacation. Blair issued a statement praising the cooperation between the two countries, which "underlines the threat we face and our determination to counter it." Some speculated that UK flights had been targeted because of Britain's military campaigns in Iraq and Afghanistan. In Wisconsin, Bush placed the blame for the plot on al-Qaida-style terrorism. "This nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom to hurt our nation," he said. Meanwhile stringent security measures have been implemented at all UK airports. Passengers will only be allowed to take essential items in clear plastic bags and nothing in pockets onto planes. Only wallets, passports, travel documents, essential medicines - not liquids unless verified as authentic - baby and sanitary items will be allowed. All passengers will be hand searched, all footwear screened and all liquid items will be taken from passengers. In a statement, the BAA said: "We wish to stress that these are precautionary measures. During the next few days airports will be extremely busy; therefore, only those intending to fly should come to the airport. Passengers are also asked to be patient while these additional security measures are put in place. Delays are likely and passengers are therefore asked to allow extra time for their journey." AP contributed to this report.