A picture taken on BBC World channel shows British Home Secretary John Reid speaking in London. Britain said that police have foiled a plot to cause "mass murder on an unimaginable scale" by blowing up planes to the United States, putting the nation on maximum terror alert and snarling air traffic around the world. (Xinhua/AFP Photo)

    LONDON, Aug. 10 (Xinhua) -- British Home Secretary John Reid said on Thursday that he believed the main players in the foiled plot to blow up airplanes in mid-flight from Britain to the United States "are in custody."

    At a police and government news conference in London, Reid said that 21 suspects had been arrested in London, its suburbs, and in Birmingham under the Terrorism Act. Police are still searching homes in the respective neighborhoods.

    "We are involved in a wide, deep struggle against evil people," the home secretary said, and the potential loss of life in the foiled plot was unprecedented.

    Security at airports across the country has been beefed up, and massive delays have been caused at all airports cross Britain.

    British Airways (BA) on Thursday advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in Britain.

    Passengers will be allowed to take only travel documents, eyeglasses and a handful of other items through airport security, the company said in a statement.

    London's Heathrow airport is now closed to all incoming flights. BA has cancelled all short-haul European flights as well as applying significant delays to services. Olympic Airways has grounded all flights. It is estimated that about 400,000 people are affected by the travel chaos.

    Transport Secretary Douglas Alexander has urged all passengers to show understanding.

    Police said their operation was "at a very early stage", and that they were working with foreign agencies to investigate the affair. Enditem