* British police say they have arrested 21 people.
* Interior Minister John Reid says plot involved a "number of aircraft" and police source says it is believed to have involved a "liquid chemical" device.
* Britain steps up security to "critical", which means "an attack is expected imminently".
* US Department of Homeland Security steps up security in aviation and bans carrying of liquids on airplanes, but says there is no indication of plotting within the United States.
* British Airports Authority asks airlines to cancel all in-bound flights from Europe into London's Heathrow airport temporarily. It also warns of severe delays due to extra security measures and asks passengers to stay away from Heathrow.
* Oil prices fall 1%, pounds slips against the euro and the dollar and European shares slide on the news.
