Police are hunting a man who blocked his victim's path before grabbing her neck and arm and sexually assaulting her in Melbourne's south-east.

The terrifying sex attack occurred about 9pm on Monday, as the 30-year-old victim walked east on Heatherton Road towards Dunblane Road in Noble Park.

Detective Sergeant Wayne Amor said that as the woman passed a medical centre at the intersection of the two roads, the offender tried to engage her in conversation.

When the victim tried to back away, the attacker - described as Indian in appearance, about 183 centimetres tall and aged in his early to mid 20s - blocked her path before grabbing her neck and hand and indecently assaulting her.

The woman fell to the ground while the offender continued to sexually assault her.