American actor Johnny Wactor has been shot dead in Los Angeles by a thief who was trying to steal a part from his car.

The 37-year-old suffered fatal injuries after interrupting three men who were attempting to remove the catalytic converter from his parked vehicle.

Wactor - who previously featured in the US soap opera General Hospital - was pronounced dead early on Saturday.

His agent David Shaul told Variety he was an "outstanding human being".

He continued: "He would literally give you the shirt off his back.

"After over a decade together, he will leave a hole in our hearts forever.”

Wactor's mother Scarlett told NBC4 her son was returning to his car after work and discovered a man tampering with it.

Believing it was being towed and not suspecting danger, he approached the man to ask what was happening, then was immediately shot, she said.

She told the outlet the suspect fled with two other men, and described her son's killing as "senseless".

The incident happened at the corner of West Pico Boulevard and South Hope Street in downtown LA. No arrests have been made.

Wactor’s brother, Grant, told the LA Times reported that Wactor had been working as a bartender on Saturday and was walking a co-worker to her car after when he was killed.

"He was... one of the most charismatic people I knew. Because when he talked or listened, you could tell it was genuine,” his brother said.

“He lived life his way. He did exactly what he wanted, even to his last day. He walked the walk.”

A tribute shared by producers at General Hospital said South Carolina-born Wactor was "truly one of a kind and a pleasure to work with each and every day".

As well as a two-year stint on the show as Brando Corbin, Wactor also starred in the 2013 NBC series Siberia and featured in two episodes of HBO's Westworld.

Catalytic converters are valuable targets for thieves as they contain precious metals which can be sold.