Prosecution says 83-year-old captured in 2011 was at center of 'murder and mayhem' in Boston for almost 30 years

James "Whitey" Bulger was at the center of "murder and mayhem" in Boston for almost 30 years, a federal prosecutor said as the alleged mobster's racketeering trial began in the city on Wednesday.

Prosecutor Brian Kelly told jurors that Bulger led the violent Winter Hill Gang, which "ran amok" in Boston for nearly three decades, killing 19 people, extorting millions from drug dealers and other criminals and corrupting police and FBI agents.

"At the center of all this murder and mayhem is one man – the defendant in this case, James Bulger," Kelly said.

Bulger's lead attorney, JW Carney Jr, went after the prosecution's star witnesses, including the hit man John Martorano, who admitted killing 20 people and has agreed to testify against Bulger. Martorano served 12 years in prison for his crimes, in what Carney called an "extraordinary benefit" for his cooperation with prosecutors.

"The federal government was so desperate to have John Martorano testify … they basically put their hands up in the air and said, 'Take anything you want,'" Carney said.

Other once-loyal Bulger cohorts who will likely testify against him include Stephen "the Rifleman" Flemmi, his former partner, and Kevin Weeks, a former lieutenant who led authorities to six bodies. The government plans to show the jury a 700-page file it says shows that Bulger, while committing a long list of crimes, was also working as an FBI informant, providing information on the New England Mob – his gang's main rivals – and corrupting FBI agents who ignored his crimes.

Kelly said Bulger's gang succeeded by instilling fear in other criminals and corrupting law-enforcement officials who tipped them off when they were being investigated. "It was part of a strategy they had, and it worked for them," he said.

Carney denied that the FBI ever tipped off Bulger, saying: "James Bulger never, ever – the evidence will show – was an informant." Carney acknowledged that Bulger was involved in illegal gambling and drugs but told the jury that Bulger paid law enforcement to protect him from prosecution.

A 23 June 2011 booking photo provided by the US Marshals Service shows James "Whitey" Bulger. Photograph: AP

Bulger, now 83, became one of the US's most wanted fugitives when he fled Boston in 1994, after receiving a tip from his former FBI handler, John Connolly, that he was about to be indicted. He was captured in 2011 in Santa Monica, California, where he had been living with his longtime girlfriend in a rent-controlled apartment. Connolly was convicted of racketeering for warning Bulger and later of second-degree murder, for giving information to Bulger that led to the killing of a Boston businessman in Miami. Bulger's lawyers have indicated that they will argue that Connolly fabricated informant reports in Bulger's lengthy FBI file.

The defense may also present another side of Bulger that was seen by some residents of South Boston, where he was known for years as a kind of harmless tough guy who gave Thanksgiving dinners to his working-class neighbors. Prosecutors, however, plan to call one family member of each of the 19 people prosecutors allege were killed by Bulger and his gang. Among the victims were two 26-year-old women, who Bulger is accused of strangling.

The trial is expected to last three to four months.