An Afghan judge has convicted and sentenced to death four men for their role in the brutal mob killing of a woman in Kabul in March.

Judge Safiullah Mojadedi announced the sentences at Afghanistan's Primary Court in the capital as part of a trial of 49 suspects, including 19 police officers.

Judge Mojadedi sentenced eight defendants to 16 years in prison and dropped charges against 18. The remaining suspects are to be sentenced on Sunday.

The March 19 killing of the 27-year-old woman named Farkhunda in a central Kabul street shocked many Afghans and led to a nationwide movement calling for justice.

Prosecutors alleged that Farkhunda was beaten to death in a frenzied attack sparked by a bogus accusation that she had burned a copy of the Koran.

The trial, which began on Saturday, only involved two full days of court proceedings - unusually quick for a slow-moving judicial system.

The defendants have the right to appeal against their sentences. The charges included assault, murder and encouraging others to participate in the assault. The police officers were charged with neglecting their duties and failing to prevent the attack.

Farkhunda's brutal killing shocked many Afghans, though some public and religious figures said it would have been justified if she had damaged a Koran. A presidential investigation found that she had not damaged a copy of the Muslim holy book.

Her last agonising hours were captured on mobile phone cameras by witnesses and those in the mob who attacked her. The videos of the assault circulated widely on social media, showing Farkhunda - who, like many Afghans, went by only one name - being beaten, run over with a car and burned before her bloodied body was thrown into the river.

The incident sparked nationwide outrage and soul-searching, as well as a civil society movement seeking to limit the power of clerics, strengthen the rule of law and improve women's rights.

Farkhunda's parents addressed the court before the sentences were handed down, asking that the accused be dealt with according to the law.

Afghanistan's judicial system has long faced criticism for its inability to offer the majority of Afghans access to justice. Women especially are sidelined, despite constitutional guarantees of equality and protection from violence, a recent report by the United Nations concluded.

The attack on Farkhunda was widely seen as symptomatic of the general low regard for women in Afghan society, where violence often goes unpunished.

Farkhunda's brother Mujibullah said the family was angered by the leniency of the court towards the majority of the defendants.

"The outcome of the trial is not fair and we do not accept it - you saw just four people sentenced to death but everybody knows that more than 40 people were involved in martyring and burning and beating my sister.

"Eighteen people have been freed. The court should punish them and that should be a lesson for anyone who would commit this sort of crime, anywhere in our country, in the future."

Her mother Bibi Hajira told the court: "Everybody saw what happened and I insist on justice. That's all I want."