A pedestrian has died after he was struck by an illegally modified electric bike near Melbourne.

The 69-year-old man was walking on Frankston-Flinders Rd in Hastings, on the Mornington Peninsula, on a shopping trip when he was hit at 6.40pm on Monday.

Paramedics rushed to the scene and treated the man for life-threatening upper and lower body injuries.

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He was flown to Royal Melbourne Hospital in a critical condition, but succumbed to his injuries on Tuesday afternoon.

The bike rider, a 24-year-old Hastings man, was also taken to The Alfred Hospital with serious injuries.

He is expected to be interviewed by police as part of the investigation.

Police said the electric bike had been illegally modified to go faster.

“The battery has [been] increased in size,” Inspector Craig McEvoy told The Age.

“It’s been held together with duct tape and cable ties to increase the speed and the capacity, the power outage, of the actual bike.”

The modified bike is believed to be capable of reaching speeds of up to 90km/h.

Some witnesses at the scene told The Age they heard people screaming.

“I heard screams, somebody going, ‘Ahh, ahh, ahh’,” Leanne Davies said.

“It went on for a minute or so and then there was a bit more a bit later.”

Police are investigating the circumstances surrounding the collision.

Anyone with information or dashcam footage is urged to contact Crime Stoppers.