An alleged fake bomb was planted at a service station in Fortitude Valley by a man who, police said, held up the premises and fled in a taxi.

Police allege a 50-year-old man who lived in the inner Brisbane suburb put a "suspicious-looking device" next to a petrol pump at the Barry Parade service station about 1.20am on Tuesday.

He walked inside the shop and threatened the shop attendant, demanding cash, police said.

The attendant gave the man money from the register and the man left in a taxi.

Officers found him near the corner of Brookes Street and St Paul's Terrace and took him into custody.