Detectives investigating the support of alleged terrorist activity in Syria have arrested four men, Scotland Yard has said.
Three men, aged 18, 22 and 31, were held at separate residential addresses in east London.
A 33-year-old man was arrested at Gatwick airport on Wednesday as he tried to take a flight out of the UK.
The men are all in custody at a south London police station while searches at three properties take place.
Whitehall officials say the arrested men are based in the UK and are not Syrian.
The Metropolitan Police said officers from its counter-terrorism command would question the men on suspicion of the commission, preparation or instigation of acts of terrorism, an offence under the Terrorism Act 2000.
The arrests are part of an investigation into travel to Syria in support of alleged terrorist activity, it added. Two men have previously been charged in connection with the inquiry.