[JURIST] The London Metropolitan Police Service [official website] announced Thursday that it had foiled an alleged terrorist plot to simultaneously blow up several planes traveling from London's Heathrow airport [corporate website] and surrounding airports to various destinations in the United States and had arrested 21 people suspected to be involved. Metropolitan Police Deputy Commissioner Paul Stephenson [official profile] referred to the plot as a plan to "commit mass murder on an unimaginable scale," while UK Deputy Assistant Commissioner and head of the Met's Anti-Terrorism Branch [official website] Peter Clarke told the media [Met press release] that several suspects intended to hide explosives in carry-on luggage and detonate the explosives mid-flight. UK authorities acted early Thursday morning to arrest the suspects after an investigation that has lasted several months. Authorities arrested most of the suspects in London, but some hailed from Thames Valley and Birmingham.

US counterterrorism officials said Thursday that the terrorists had targeted flights on United, American and Continental airlines traveling to New York, California and Washington, DC. A senior US counterterrorism official also said that as many as 50 people were involved in the thwarted plot, adding that it had the "footprint" of al Qaeda on it. British authorities raised the threat level to "critical" [Home Office materials] indicating that an attack is expected imminently, while the US Department of Homeland Security [official website] raised the US aviation threat level to the highest level [TSA press release] for the first time since the warning system was implemented after the Sept. 11 2001 terrorist attacks [JURIST news archive]. AP has more. BBC News has local coverage.