An off-duty San Diego police sergeant accused of assaulting his girlfriend during a night out in the Gaslamp District pleaded not guilty Monday to felony domestic violence and other charges.

Oscar Armenta, a 13-year veteran of the San Diego Police Department, faces 4 1/2 years in prison if convicted of the felony count and three misdemeanor charges, said Deputy District Attorney Melissa Diaz.

Armenta -- who is out of custody on $25,000 bail -- agreed to stay away from his girlfriend and two good Samaritans who intervened, according to the prosecutor.

Police said the assault took place July 3 about 8:30 p.m. in the 700 block of Island Avenue. The extent of the woman's injuries were unclear.

Armenta, 41, was placed on paid leave pending the outcome of the case. The defendant has no prior record, the prosecutor said.

In addition to the felony count, Armenta is charged with misdemeanor domestic violence and two counts of misdemeanor battery on the good Samaritans, Diaz said.

A readiness conference is scheduled for Aug. 22 and a preliminary hearing for Sept. 6.