An Egyptian judge has ordered the detention of a policeman on trial for the January killing of a female protester taking part in a peaceful demonstration in downtown Cairo.

Yassin Hatem Salahedeen, a 24-year-old police lieutenant, is facing a manslaughter charge over the death of Shaimaa el-Sabbagh, a 32-year-old mother of a small boy.

The policeman had been free on bail, but Judge Mustafa Hassan Abdullah on Sunday ordered his detention in the trial's opening hearing.

Video clips widely shared on social media show Sabbagh collapsing in a colleague's arms with her head, chest and back soaked in blood after a masked policeman fired birdshot in her direction.

Rights lawyers and witnesses have also said the police hampered efforts to save Sabbagh's life by preventing an ambulance from passing through the cordon.

Defence lawyer Mohammed Abdel-Aziz said Sunday's hearing lasted nearly six hours, during which a video clip submitted by the prosecution as evidence was shown.

The video, according to Abdel-Aziz, showed a masked policeman firing at Sabbagh from a short distance with a shotgun, then swapping the weapon for a tear gas launcher held by a police recruit.

Salahedeen acknowledged to the judge that it was him firing the rifle in the video, said Abdel-Aziz, who represents Sabbagh's family and attended Sunday's hearing.

The court was also shown a total of 120 photographs chronicling what happened during the brief demonstration on January 24, the eve of the fourth anniversary of the uprising.

Some 40 members of a left-leaning party gathered in downtown Cairo that day with the intention of marching to nearby Tahrir Square, epicentre of the uprising, to lay wreathes in memory of the protesters who fell during the 18-day revolt.

The trial will resume May 14, when the court will begin hearing prosecution witnesses.

Egyptian police consistently maintain they do not use birdshot against protesters despite overwhelming evidence to the contrary.

In Sabbagh's case, a senior police officer insisted in a news conference shortly after her death that forensic tests showed she was shot by rounds not used by the police force - implying the involvement of an unknown third party.

The killing has struck a nerve with many Egyptians, mostly because of the wide distribution on social media of the images of Sabbagh after she was shot, being lifted off the ground by a colleague with blood running down her face.

Her death also has stoked anger over the perceived brutality of the police and called into question the validity of a law adopted in December 2013 that bans all street protests without prior permits.

Many of those who participated in the January protest, in addition to at least one witness who testified to prosecutors on the day's events, are facing a separate trial for breaking the law on street demonstrations.

That trial opened this weekend and will resume on May 23.