A 28-year-old man has been arrested by anti-terror police in Hertfordshire in connection with providing weapons linked to international terrorism.
Police said searches in east London would continue 'for some time'
Specialist Metropolitan Police firearms officers challenged the man when he got out of a parked car near the South Mimms service station, off the M25.
The man, from east London, was taken to a central London police station.
Addresses in the Waltham Forest and Newham areas of east London are being searched as part of the investigation.
The arrest, which police say is not connected to the investigation into the 7 July bombings in London, follows an intelligence-led operation by the police and the security services.
Police said no shots were fired when the man was challenged and then arrested at about 1130 GMT on Friday.
Armed officers were joined by other specialists from the Met and local officers from Hertfordshire Police.
The Met Police said in a statement that the two searches were "likely to continue for some time".
No hazardous materials had been found at the addresses, the statement added.