Two teenagers have been arrested on suspicion of terrorism offences as part of an ongoing investigation into alleged Islamist extremism.
The Metropolitan Police said an 18-year-old man was arrested in Essex on Wednesday on suspicion of encouraging terrorism.
He remains in police custody after a warrant was granted to allow detectives to question him for a further six days.
A 17-year-old girl was arrested on Friday at an address in east London.
On Wednesday, a 13-year-old boy was arrested in west London on suspicion of sharing Islamist terrorist material.
The force said all three arrests were made as part of a wider investigation into alleged Islamist extremism.
Cdr Richard Smith, head of the Met's Counter Terrorism Command, said: "We have seen three teenagers arrested this week by Met officers for terrorism offences, albeit as part of separate investigations.
"We need to let this investigation run its course, but it is a further indication of a concerning upward trend in police action against younger people for terrorism-related matters."
He remains in police custody after a warrant was granted to allow detectives to question him for a further six days.
A 17-year-old girl was arrested on Friday at an address in east London.
On Wednesday, a 13-year-old boy was arrested in west London on suspicion of sharing Islamist terrorist material.
The force said all three arrests were made as part of a wider investigation into alleged Islamist extremism.
Cdr Richard Smith, head of the Met's Counter Terrorism Command, said: "We have seen three teenagers arrested this week by Met officers for terrorism offences, albeit as part of separate investigations.
"We need to let this investigation run its course, but it is a further indication of a concerning upward trend in police action against younger people for terrorism-related matters."