Robbers grabbed a paving stone from a church wall and hurled it through the locked door of a garage in Brighton.
They climbed through the hole, threatened the cashier with a knife and fled with the contents of the till.
The raid happened just after midnight last night at the Sussex Square service station in Eastern Road.
The cashier suffered shock but was otherwise unhurt.
Police today were studying CCTV footage taken in the garage at the time.
The man wielding the 12in-bladed knife was in his early 20s, 5ft 8in, and wore a grey hooded top.
His accomplice was about 16, had fair hair and wore a light or white-coloured top.
The pair escaped in a car driven by a third robber.
Witnesses or anyone with information should contact Detective Inspector Fiona Grant at Brighton and Hove CID on 0845 6070999.
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