HAYWARD -- A man on crutches beat and seriously injured another man with one of his crutches after the man tried to rob him and his girlfriend along a South Hayward street early Thursday, police said.

The chain of events that police called an ironic role reversal began just before 12:30 a.m. as the couple walked along the 29000 block of Mission Boulevard, Sgt. Mark Ormsby said.

Officers were called to the scene for a report of an armed robbery. When they arrived, a 34-year-old man and his 21-year-old girlfriend told them they had just left the All Suites Islander Motel on Mission Boulevard when a man who appeared intoxicated walked up to them and told them he had a gun.

The couple, both from San Francisco, told officers that the alleged robbery suspect reached underneath his jacket and into his waistband and then demanded money.

That's when the man grabbed one of his crutches and hit the suspect in the face, causing a serious head wound, Ormsby said.

The suspect was taken by ambulance to St. Rose Hospital to be treated for his injuries, police said.

A weapon was never seen, police say.

The couple were not injured, and the suspect was arrested on suspicion of attempted armed robbery.

Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund.