Writer Irvine Welsh paid tribute to his "incredible" friend Bradley Welsh after the star of Trainspotting 2 was killed in Edinburgh.
The actor, who also appeared in an episode of Danny Dyer's Deadliest Men, died after an incident on Chester Street on Wednesday night.
Police were called to the scene around 8 pm and found a man with serious injuries.
He died at the scene and the police are treating the suspect as death.
Welsh (42) starred alongside Ewan McGregor, Ewen Bremner, Jonny Lee Miller and Robert Carlyle in the T2 Trainspotting, playing the gangster character Mr Doyle.
Irvine Welsh said her heart was broken with the news.
The writer tweeted, "Bradley John Welsh, my heart is broken. Goodbye my beautiful and wonderful friend.
"Thank you for making me a better person and helping me see the world in a gentler, wiser way."
The police are asking anyone with information to contact them immediately.
A Scottish police spokesman said: "Death is being treated as a suspect and investigations continue.
"Officers will continue to provide a high visibility presence in the area to engage with the public, provide security and gather information that may assist with this investigation."
Welsh appeared in Danny Dyer's Deadliest Men in 2008, where he described himself as a "born leader" in the episode.
The actor later became involved in charitable projects in Edinburgh, including helping young people get away from a life of crime through their Holyrood Boxing Gym.
He was supposedly a former British boxing champion ABA Lightweight.
Other people also paid tribute to social media, with a woman posting a photo of herself and friends alongside Welsh in a boxing ring.
She said: "You are a treasure to me, you are dear to me. Our coach, our motivator, but more importantly, our friend Brad Welsh.
One man described Welsh as a "huge character" in Edinburgh.
He said: "Devastating news about Brad Welsh tonight, a huge character in Scottish amateur boxing and Hibernian support and someone who has contributed a lot to society through his charity work and boxing gym. REST IN PEACE."
Writing on social networks, local resident Alasdair Morton said armed police were "everywhere" in the area.
He tweeted, "Armed police across Chester Street. Road closed. We were told to enter. Before we did, we heard the police tell the national incident response unit – the shot would be (sic) in the head. "- PA