Four people have been arrested on suspicion of "failing to disclose information which might be of assistance in securing the apprehension, prosecution or conviction of a person involved in an act of terrorism".
Two men, aged 21 and 31, and a 25-year-old woman were arrested by counter-terrorism officers at two residential addresses in east London, and a 25-year-old man was arrested at a residential address in Lancashire, Scotland Yard said.
The arrests come after two brothers who were stopped by police in Dover appeared in court last month accused of travelling to a terrorist training camp in Syria.
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The Metropolitan Police has paid £45,000 in damages in an out-of-court settlement following the 'unlawful arrest' of a black motorist.
John Cantlie, who worked for a number of Fleet Street newspapers, is thought to have been taken hostage in November 2012.