Peter Clarke, head of Anti-Terrorist Branch - opens by saying we must be careful not to be prejudiced against any group.
Plot to conceal explosive devices - devices constructed in UK. Timing of plots subject to investigation.
Today marks the combination of an investigation that's lasted several months and will last long into the future.
Clarke: The plot had global dimensions Investigation reached a critical point last night when decision was made to take emergency action. Throughout the night significant arrests made. People arrested remain under custody in London.
Searches also underway at business and residential locatoins and will take some time to complete.