The operation is understood to relate to a possible plot involving Islamist extremists with potential targets in the UK.
The threat was not thought to be imminent.
Some of those arrested are understood to be British nationals.
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John Cantlie, who worked for a number of Fleet Street newspapers, is thought to have been taken hostage in November 2012.
The union published a 'no show' league table for last year the worst offenders for cancellations.
Zalkalns was jailed in his native Latvia for eight years in 1988 after he admitted killing his wife with a piece of electrical equipment.