Jurors are expected to hear more grisly details about reputed mob boss James "Whitey" Bulger's reign of terror in Boston when a former hit man testifies against his ex-boss.

Mr Bulger faces a 32-count indictment that includes 19 murders he allegedly committed or ordered.

The 83-year old, known as "Whitey" for the shock of blonde hair he had even as a child, is accused of running Boston's "Winter Hill Gang" for decades.

He is also accused of threatening to kill small-time criminals unless they paid protection money.

He has pleaded not guilty to all counts.

The trial marks the final chapter in one of Boston's longest-running crime dramas.

It will bring some closure to families of the 19 murder victims who are planning to come to the waterfront courthouse daily for the trial that is expected to last three to four months.

In last week's opening statements, prosecutors portrayed Mr Bulger as a hands-on killer.

Mr Bulger's lawyer described him as a mild-mannered criminal who had engaged in illegal gambling, loansharking and drug dealing but not murder.

Jurors heard from two former bookmakers who made illegal bets on sports events and will hear more from one of them, Richard O'Brien, today.

Mr Bulger's lawyer will then cross-examine him.

The police started to cultivate them as informants for leads on Mr Bulger in the 1980s when officials tried to crack down on violent gangs in Boston.

Last Friday the 84-year-old Mr O'Brien, who had spent decades placing illegal bets on horse-racing and football games, told jurors of Bulger's threats.

On Monday, the prosecution will also put on one of its star witnesses, John Martorano, who admitted to killing 20 people and served 12 years in prison.

Mr Bulger's attorneys have raised questions about the reliability of the testimony of Mr Martorano and Mr Bulger associates Stephen Flemmi and Kevin Weeks who are also expected to take the stand.

The men, Mr Bulger's lawyers contend, fingered the mobster for murders he did not commit in exchange for lesser sentences for their own crimes.

Mr Bulger fled Boston in 1994 after being tipped off that he would soon be indicted.

In his 16 years on the run he was on the FBI's Most Wanted List of criminals. He was captured in 2011 in California.