James Ainsworth is said to have been set upon during a break in his trial at the High Court in Glasgow on Wednesday afternoon.
As reported in the Evening Times, the 61- year-old received CPR in the street as paramedics battled to save his life. Ainsworth was rushed to the city's Southern General hospital where he remains in a critical condition.
A suspect is now in custody and a judge has ordered a full inquiry into the incident.
Ainsworth, of Largs, Ayrshire, was on trial accused of a string of offences against a number of individuals dating back to the 1960s.
Prosecutor Kath Harper yesterday moved to halt the trial stating it "seemed unlikely" that Ainsworth would be fit to attend court "in the near future at all".
Miss Harper also said: "Mr Ainsworth is in intensive care at the Southern General. His injuries are life threatening and he remains unconscious.
"He was put into a medically induced coma. Sedation was lifted and doctors are awaiting a reaction.
"Medical staff are unable to give any prediction for recovery."
The jury was discharged after Judge Lady Scott said Ainsworth had to be taken to hospital for an "emergency admission".
She added: "This may come as a shock and I hope you do not find it too distressing."
The judge earlier stated "security issues might arise" after what took place.
Prosecutors can re-raise proceedings against Ainsworth if he is fit to return to the dock in future.
Police Scotland said a 31-year-old man is in custody and is due to appear at Glasgow Sheriff Court today.