A 35-year-old Quincy man accused of a “nightmarish attack” on a woman in the South End Saturday night before a Good Samaritan stepped in to defend the victim has been ordered held without bail, according to the Suffolk County District Attorney’s Office.

Amos Sykes is facing charges of indecent assault and battery on a person 14 or over, and assault and battery (injuries) following the weekend’s alleged attack in the area of W. Canton Street and Appleton Street.

Sykes is accused of punching a woman in the head as he sat on top of her, several witnesses told Boston Police. The victim was screaming during the assault, the witnesses told police.

When police officers arrived at the scene, they saw a Good Samaritan fighting with Sykes, who then tried to get away from the cops. Sykes was quickly apprehended.

According to Boston Police, the woman before the attack had been walking on Columbus Avenue when the suspect grabbed her from behind, forced her onto the ground and began punching her in the head.

Sykes tried to take her clothes off before a witness stepped in to fight off Sykes. Boston EMS transported the victim to a local hospital with non-life threatening injuries.

“This was a nightmarish attack that left the victim bloodied and bruised, and would likely have resulted in an even worse outcome if not for the extraordinarily brave intervention of a Good Samaritan,” Suffolk DA Kevin Hayden said following Sykes’ arraignment on Monday.

“In cases like this the victim’s injuries often transcend the physical,” Hayden added. “Our office has advocates specially trained to treat this type of trauma, and they will be by this victim’s side throughout the court process and after. I thank Boston Police for their quick response and arrest and I am especially grateful for this Good Samaritan’s courageous action.”

Sykes was ordered held without bail pending a dangerousness hearing on Wednesday.

Detectives from District D-4, along with the Boston Police Sexual Assault Unit, will continue to investigate the incident.