A former Oklahoma City police officer has been found guilty of crimes including rape and sexual battery by a jury in a case where prosecutors said he preyed on women who had trouble with the law, hoping their word would not stand up against his.

Daniel Holtzclaw, who turned 29 Thursday, broke down in tears as the verdict was read. He was charged with sexually assaulting and raping 13 women and found guilty of 18 of the 36 charges, including four of the six rape charges.

Sentencing was set for January and he could face life in prison.

"I didn't do it," said Holtzclaw, before he was led out of the courtroom in handcuffs.

The jury has been deliberating since Monday night.

The allegations against Holtzclaw brought new attention to the problem of sexual misconduct committed by law enforcement officers, something police chiefs have studied for years.

In closing arguments on Monday, prosecutors said Holtzclaw targeted his victims by going after women he came across while on patrol. He ran background checks and went after those who had outstanding warrants, previous arrests or carried drugs or drug paraphernalia.

Protestors carry signs outside the courthouse as the jury deliberates in the trial of Daniel Holtzclaw, a former police officer accused of sexually victimizing 13 women in the neighbourhood he patrolled. (Sue Ogrocki/Associated Press)

They said he did this because he did not think authorities would take the victims' word over his if he had to defend himself against sexual assault allegations.

"He exercised authority on those society doesn't care about," Assistant District Attorney Gayland Gieger said in closing arguments.

The defence said the victims provided testimony that was unreliable and dishonest.

Defence attorney Scott Adams said Holtzclaw was an honourable police officer whose activities were made to appear evil and suspicious.

Thirteen women took the stand in the trial, which began more than a month ago, telling jurors of sexual assaults that ranged from touching over their clothing to forced oral sex and rape.

All of the accusers were black. Holtzclaw is half-white, half-Japanese. Some supporters of the women questioned whether the all-white jury would fairly judge their allegations.

One of the final people to testify was a girl who said she was 17 at the time of the sexual assault and Holtzclaw raped her on her mother's front porch.

Holtzclaw, who did not testify, was fired over the accusations in January 2015 after approximately three years on the job.