Two teenagers have been arrested on suspicion of terror offences, Scotland Yard has said.
An 18-year-old man was arrested at an address in Essex on Wednesday and remains in custody.
He is being questioned on suspicion of encouraging terrorism and police have been granted permission to detain him until Wednesday.
A 17-year-old girl was arrested in east London on Friday under section 41 of the Terrorism Act 2000 and has been taken to a London police station.
Both are being questioned as part of the same investigation into extreme Islamist ideology.
Commander 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.
"Across the country, police are working hard to try and prevent young people from being radicalised, and acting quickly when it is suspected that offences have been committed."
He appealed for people to report suspicious behaviour, saying: "Trust your instincts and contact police."
To report possible terrorist activity contact gov.uk/ACT or the anti-terrorist hotline on 0800 789 321. In an emergency, dial 999. Click here for information on how to help anyone you suspect may be being radicalised.