Boston police Officer Joaquim Antunes arrived around 9:45 a.m. yesterday on a Dorchester street closed for a potential gas leak when he heard a man's loud, angry voice and the fearful cries of a woman.

Then the 10-year police veteran, about to start his paid detail, heard something else.

"You could actually hear the smacking noise," Antunes said, slapping one hand against the other. "It sounded as if someone was being beaten."

Antunes got on his radio to call for backup, then ran into the three-story Quincy Street apartment building, ringing all three buzzers at the front door. Antunes was soon joined by Officer Richard J. Driscoll, who had sprinted over from a detail post a block away in response to his call for help.

Peter Scrima, a truck driver and laborer for Riley Bros. construction, was working nearby and said the police presence was fortunate.

"She got lucky," he said of the woman. "How often do you have two cops standing outside your door. It doesn't happen too often, I don't think."

After some moments of ringing bells, the front door opened and Nkrumah Hartfield, 40, told officers he had been arguing with his girlfriend, they said. Other officers, who had responded to the call for help, held Hartfield while Antunes and Driscoll went to the third-floor apartment.

There they found the woman, whose name was not released by po lice, sitting in the kitchen, holding her 18-month-old daughter, a welt rising above her left eye.

"She looked scared, scared and kind of relieved," Antunes said.

Hartfield slammed her in the head with a telephone, she said, and the force of the blow was so powerful it knocked her and her daughter out of the chair where they were sitting, according to the police report.

"She said she had just gone through some domestic violence," Driscoll said. "She said she was trying to get away from it."

After the woman told Antunes and Driscoll her account of what had happened, police arrested and handcuffed Hartfield, who slammed his head into a cruiser and tried to kick out its windows, police said.

Hartfield was taken to Beth Israel Deaconess Medical Center for treatment. He is scheduled to be arraigned Monday in Roxbury Municipal Court, where he will face charges of two counts of domestic assault and battery and one count of assault and battery with a dangerous weapon.

The woman told police that she and Hartfield started arguing Thursday night about how she disciplines her children and that the dispute resumed yesterday morning. Hartfield was on the telephone with his mother, the woman said, when he became enraged and hit her in the head with the telephone, according to the police report.

The woman and her child were taken to Boston Medical Center for evaluation, police said.

Boston Police Commissioner Edward F. Davis applauded Antunes's quick reaction.

"If the officer hadn't called for help and begin to bang on the doors and let the assailant know he was being pursued, then anything could have happened," he said. "We are just very thankful he overheard the incident and took aggressive steps to deal with it."