A prosecutor accused a former Rikers Island jail guard of abusing his power by kicking inmate Ronald Spear to death in 2012, but the defense disputed the cause of death as Brian Coll’s trial on civil rights charges began Friday in Manhattan federal court.

“Two correction officers held Mr. Spear on the ground,” prosecutor Jeannette Vargas told jurors in a graphic opening argument. “This defendant was wearing heavy work boots. He kicked him. He kicked him in the head. Then he kicked him again and again.”

Coll, 47, of Smithtown, faces up to life in prison if he is convicted of civil rights violations that resulted in death, and also faces cover-up charges. The family of Spear, who was at Rikers awaiting trial on burglary charges, attended the start of the trial.

Vargas said Spear suffered from diabetes, kidney problems and a diseased heart. He was held in the jail infirmary, got into a dispute with 10-year veteran Coll over seeing the doctor, and started the fight by kicking him and trying to push past into the doctor’s office.

But she said the crime occurred after Coll responded by punching Spear and with the help of other officers subduing him on the ground. He then, despite calls from other officers to stop, kept booting him in the head and afterward — with Spear’s eyeballs rolled up into his head and his mouth agape — taunted Spear and banged his head against the floor.

“It’s about an officer who was put in a position of power over others and how he abused that power to violate that man’s civil rights, how [he] abused his power to savagely kick that man to death,” Vargas told the jury.

Coll’s defense lawyer Josh Dratel called the case a “tragedy,” but said jurors would hear “inconsistent” testimony about the kicking and emphasized the prosecution’s admission that Spear was a “difficult” inmate and that Coll did not start the fight.

“This is not a case alleging some calculated, unprovoked attack on an inmate,” he said. “This is about an altercation started by the inmate himself ... When an inmate attacks an officer, the use of force is appropriate.”

He also challenged the government’s claim that the alleged blows to the head instead of the preceding legal efforts to restrain him were the cause of the stress that led to Spear’s death from cardiac arrest, calling it a “convoluted” and speculative assertion.

“You will not hear any doctor say conclusively that is what occurred,” Dratel said. “It is a theory ... They’re going to try to pull various rabbits out of hats.”

Prosecutors said they would call two witnesses involved in the incident and alleged cover-up — one a guard who was granted immunity, and another guard who has pleaded guilty and is hoping for leniency at sentencing.

The trial is expected to last two weeks.