A man wearing a clown mask and carrying a gun has terrorised pedestrians in Melbourne's west during a bizarre series of early-morning drive-bys.

Police spokeswoman Melissa Seach said authorities were aware of three incidents where people were threatened by a masked man in St Albans and Sydenham on Tuesday.

The spree began about 5.30am when a white car containing two men pulled over beside a woman on Overton Lea Boulevard in Sydenham.

One of the men, wearing a clown mask and carrying a firearm, got out of the car and began walking towards the woman.

However, he was disturbed by a passing car and fled the area.