Armed thieves tied up a woman and then hit a policeman in the head with a hatchet in Melbourne's east on Thursday.

Police were called to a robbery at a cigarette store on Station Street, Ferntree Gully, about 2.45pm, and arrived to find two men armed with an iron bar and a hatchet.

Victoria Police spokeswoman Melissa Seach said they had tied up a woman and were trying to rob the store.

"When police arrived, an altercation occurred and one of the members was struck to the head with a hatchet," Leading Senior Constable Seach said.