Three teenagers who were stopped from travelling to Syria from Turkey have been flown back to the UK and arrested, Scotland Yard has said.
Two boys aged 17 from north-west London and a man, aged 19 were returned to the UK on Saturday night, the Met said.
They have been arrested on suspicion of preparing terrorist acts and are being held at a police station in London.
They were reported missing on Friday and were detained after UK police alerted Turkish officials, it added.
Scotland Yard said counter-terrorism officers had initially been made aware that the two 17-year-old boys had gone missing and were believed to be travelling to Syria.
Enquiries revealed they had travelled with a third man, police added.
"Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria. They remain in detention in Turkey," a police spokesman added.
They were returned to the UK at about 23:10 GMT on Saturday and were arrested by counter-terrorism officers.
BBC correspondent Andy Moore said the development comes after "recriminations" between UK police and Turkish officials following the disappearance of three London schoolgirls.
Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16 - all from London - took flights to Istanbul last month, from where it is feared they travelled to join Islamic State militants in Syria.
'Warning was raised'
Their disappearance led to criticisms from Turkey's deputy prime minister, who said officials in the country had not been given enough warning about their disappearance.
"On this occasion it seems that the warning was raised in the UK and that was communicated very quickly to Turkey," our correspondent said.
It comes as a fourth missing schoolgirl - who was a friend of the three London schoolgirls - was named as Sharmeena Begum.
The 15-year-old, from Bethnal Green, is thought to have left the UK in December to join Islamic State (IS) militants.
The Home Office says there are about 600 Britons "of interest" in Syria.
The BBC understands about 100 Western volunteers - including some Britons - are fighting as part of the 30,000-strong Kurdish forces, while more than 500 Britons are believed to have travelled to join Islamic State militants.
Labour MP Keith Vaz, chairman of the Home Affairs Select Committee, said the flow of young people from the UK to Syria, via Turkey, was "on a much larger scale than we envisaged".
"I welcome the action that has been taken by the Turkish authorities," he said.
"We need to be vigilant. Clearly this flight of young people to Turkey in order to go to Syria is on a much larger scale than we envisaged."