A second arrest has been made as part of the investigation into the terror attack on a Tube train at Parsons Green on Friday.
A 21-year-old man from Hounslow in west London is being held under section 41 of the Terrorism Act, police have said.
He is being held in custody at a police station in south London.
An 18-year-old man was arrested on Saturday morning at the port of Dover on suspicion on preparation or instigation of acts of terrorism.
The failed attack injured 30 people during Friday rush hour.
The threat level has been raised to critical, meaning a further attack may be imminent, security services believe.
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