A man has been arrested on suspicion of preparation of terrorist acts, the Metropolitan Police has said.
The 19-year-old was arrested in Bishop's Stortford, Hertfordshire, on Wednesday at 18:57 BST (5.57am NZ time)
An address in the town is being searched, and the man is being questioned at a south London police station.
Police say the activity is connected to the arrest of an 18-year-old man by armed police in north London on Friday.
The 18-year-old man remains in custody, and a 20-year-old woman was also arrested in south London for failing to disclose information about acts of terrorism on Wednesday.