The man was found bloodied and unresponsive after an incident about 4.15pm Tuesday on the west side of the Hopkins River bridge and was transported by ambulance to the Warrnambool Base Hospital with a range of injuries, including serious head injuries.

He was later flown to the the Royal Melbourne Hospital in a critical condition.

A Royal Melbourne Hospital spokeswoman said on Wednesday morning the victim remained in a critical condition.

A Warrnambool man aged in his mid 30s was arrested near Proudfoots late Tuesday afternoon in relation to the alleged assault.

He remains in the Warrnambool police station cells, but has not yet been charged.

It's expected he will be charged and appear in Warrnambool court later today.

More to come.

Last night: A 45-year-old Warrnambool man suffered life-threatening injuries when he was assaulted after going to a shop in east Warrnambool to get a drink late Tuesday afternoon.

The local resident was assaulted near the Hopkins River Bridge about 4.30pm and has been taken to Warrnambool Base Hospital for assessment and treatment.

He was expected to be flown to a Melbourne hospital overnight.

The alleged offender was arrested near Proudfoots.

Police Sergeant Pat Day said there's no immediate cause for residents to be concerned, but people who live on the east of the river have been asked to avoid the area and go through Allansford to make their way home.

The bridge is expected to be closed for some time.

Victorian Police Homicide Squad has been notified and is being kept abreast of the investigation as it develops.

An Ambulance Victoria spokeswoman said the man was taken to hospital in a critical condition with upper body injuries.

Two crime scenes have been established around the Hopkins River precinct and at least six police vehicles are on scene.

The main site appears to be on the immediate west side of the Hopkins River Bridge.

A police guard has been set up at the secondary crime scene at the Proudfoots car park where the alleged offender was arrested.

State Emergency Services crews have arrived on scene and have set up tents and lighting.

Sergeant Day said it was not known what prompted the assault.

"The victim, a local resident, has gone to the shop to get a drink and been seriously assaulted," he said.

"He's suffered extensive and life-threatening injuries, including head injuries.

"The alleged offender was arrested near Proudfoots. We are still in the very early stages of the investigation."

Sergeant Day apologised for the inconvenience to local residents, but said he expected the bridge to remain closed for an extended time.

Have you signed up to The Standard's daily newsletter and breaking news emails? You can register below and make sure you are up to date with everything that's happening in the south-west.