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.
More top news
The Post Office's abandoned underground railway opens its doors on Friday some 14 years after the last letter was transported through it.
Police investigating the Grenfell Tower fire have "reasonable grounds" to suspect corporate manslaughter may have been committed.
Charlie Gard will "inevitably" die shortly after being moved to a hospice and having life-support treatment withdrawn.