Counter terrorism detectives arrested a man, 56, from east London at around 5.40am this morning (July 17) after he arrived on a flight from Canada.
A Canadian national, 28, was also arrested at Heathrow Airport at around 12.35pm after he arrived by plane from the same country.
The men are being held under section 41 of the Terrorism Act 2000 at a west London police station, police said.
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Both were arrested on suspicion of membership of a proscribed organisation, contrary to section 11 of the Terrorism Act 2000.
Police searches of three addresses in east London are ongoing.