Yobs posted fireworks through the letter box of council offices in Carshalton.
Fire crews from Croydon extinguished the blaze which caused minor damage to the inside of the council-owned building which backs on to Grove Road behind the Charles Cryer Theatre.
Police are studying CCTV footage to try and identify the culprits following the incident at 8pm last night.
A fire brigade spokesman said the damage could have been much worse and urged people to take care using fireworks after a busy night dealing with firework-related calls.
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