Counter-terror police have arrested a teenager in Hertfordshire on suspicion of preparing acts of terrorism.
The 19-year-old man was detained in a street in Bishop's Stortford just before 7pm on Wednesday.
Scotland Yard said it related to the arrest of an 18-year-old man by armed police in a north London street on Friday.
Both teenagers are being held in custody on suspicion of preparing acts of terrorism, under Section 5 of the Terrorism Act 2006, the Metropolitan Police said.
A property in Bishop's Stortford is being searched.
A 20-year-old woman was arrested in south London just after 8am on Wednesday morning in connection with the case.
She is also in custody on suspicion of failing to disclose information about acts of terrorism, contrary to Section 38B of the Terrorism Act 2000.