Police cordoned off an area around the Golders Green Beth Hamedrash – popularly known as Munks – where a man outside was reported to be posing a threat on Tuesday lunchtime.
Barnet Police said no members of the public or officers are believed to have been harmed and that a man was arrested on suspicion of a number of offences which are not thought to be terror-related.
In a statement on Tuesday evening, police said they had been called to a report of "a domestic incident" in Golders Green Road around noon and followed a man driving away in a vehicle.
When he stopped close to a synagogue in The Riding, they put in place a protective cordon. The man was then believed to have "set fire to an object while officers were engaging with him".
Although he ran from the scene, he was quickly detained around 2pm and taken into custody.
No member of the public or police was reported to have been injured and the incident is not "believed to be terrorism-related", police said. Enquiries continue.
Superintendent Louis Smith, from the Met’s North West Command Unit, said: “I know this will have been a very visible and frightening incident for the local community, but please know my officers will be out in the area.
“Do talk to them if you have any concerns, we are here to listen and reassure. We will work out what happened as we continue to investigate but for the moment this looks like a self-contained incident."
Local councillor Alex Prager had earlier tweeted that the incident "appears to have involved a molotov cocktail next to a synagogue".
Golders Green Road is closed due to a security incident. Police and fire brigade on site. Appears to have involved a molotov cocktail next to a synagogue on The Riding.
— Cllr Alex Prager (@AAPrager) February 2, 2021
CST is aware of an ongoing incident in Golders Green that emergency services are attending and appear to have contained. We are working with police to determine the nature of the incident, which is currently unclear. Please avoid the area whilst the police manage the situation.
— CST (@CST_UK) February 2, 2021