MIDDLETON - A man prosecutors believe was one of the instigators of a violent melee at Middleton Jail in June was sentenced to six to eight years in state prison yesterday.

Juan Rivera, 25, of Lynn pleaded guilty to assault and battery with a dangerous weapon - a metal waist chain he used to attack a guard in the early morning hours of June 7.

That guard suffered a serious head wound and was one of five guards injured during what prosecutor Michael Patten called "essentially a riot."

Rivera was one of seven inmates in the administrative segregation unit of the jail, an area where high-risk inmates are housed. Inmates commonly refer to the area as "the hole."

The inmates were brought to the shower area and unshackled, then locked into stalls. Patten said yesterday that the men simultaneously crawled under the doors of the shower stalls and began trashing the area.

The inmates grabbed phones and computers, ripping them from their cords, and threw them at guards, raided a cleaning closet and armed themselves with broomsticks and mop handles, and grabbed shackles and handcuffs the officers had just removed from the men before their showers.

Rivera also threw a computer monitor at two guards, leading to an assault and malicious destruction charge. He also pleaded guilty to assault and battery in connection with an attack by other inmates on a guard, under a theory of joint venture.

Patten said all the inmates appeared to act in concert, and none stepped forward to intervene when the officers were injured, making all of them responsible.

The sentence - the longest so far meted out to any of the seven defendants - was the result of a plea agreement accepted by Salem Superior Court Judge Howard Whitehead.

His sentence will overlap with a term he is already serving, extending his time in prison by at least three years.

Defense lawyer John Appruzese said his client, who is already serving a three- to five-year prison term for an armed robbery in Bristol County, wants to use his time in prison to earn his high school equivalency degree and attend barber school.

Rivera is currently serving his time at Cedar Junction in Walpole.

Another leader of the melee, Eddy Lopez of Lawrence, is serving five years for his role in the incident but is serving that concurrently with a life term for murder and a federal prison term. Jurell Laronal of Dorchester pleaded guilty last month and received a 21/2- to four-year prison term. Alexis Maldonado of Haverhill received an 18-month prison term. Two others, Raul Carraquillo Llanos and Miguel Diaz, both of Lawrence, are still awaiting trial.