Two women have been arrested on suspicion of terror offences in separate police swoops.
The first, aged 26, was held at Heathrow Airport at around 9.20am after officers stopped her as she tried to board a flight to Istanbul in Turkey from Terminal 5.
She was arrested on suspicion of being involved in the commission, preparation and instigation of acts of terrorism and a "large quantity" of money was seized.
Five hours later a second woman, aged 27, was held on suspicion of the same offence in north-west London and taken into custody.
Both women are now being questioned.
Scotland Yard said the arrests were intelligence-led rather than in response to an immediate threat or risk.
Two addresses in north and north-west London are now being searched.
Both women are British citizens and are not part of the same family.