By Yasmine Saleh and Asma Alsharif
CAIRO (Reuters) - Egypt's prosecutor general on Tuesday ordered the trial of four police officers charged with killing 39 Muslim Brotherhood members in August after they were picked up in the crackdown that followed the military overthrow of President Mohamed Mursi.
The Islamists died after teargas was fired into the back of the police van in which they were being held. The four policemen have been arrested and charged with murder and unintended injury, a security source said.
Also on Tuesday, hundreds of students demanding Mursi's reinstatement protested for a fourth consecutive day at universities in several provinces.
Security forces fired teargas at supporters and opponents of Mursi in front of Mansoura University, north of Cairo, a witness said.
Riot police also surrounded an Islamic studies college in Egypt's second city of Alexandria where female students were protesting.
Egypt has been in turmoil since the Islamist Mursi, its first freely-elected leader, was ousted on July 3 amid mass protests against his rule.
Hundreds of Islamists have been killed and nearly 2,000 people, including Mursi and Brotherhood leaders, arrested since then in a concerted campaign to crush the movement.
The Muslim Brotherhood accuses the military-backed authorities of flagrant human rights abuses in the crackdown but the state denies this, describing the Brotherhood as terrorists bent on destabilizing the country.
The incident involving the dead detainees took place on August 18, four days after security forces broke up pro-Mursi protest camps in one of the bloodiest days in Egypt's recent history.
"Investigations have proven that the officers transported 45 prisoners inside a transport vehicle that is not suitable to carry more than 24 people and fired teargas inside," the prosecutor general's office said in a statement.
"This led to the death of 37 prisoners and the suffocation of two others."
The Interior Ministry said at the time the prisoners were trying to escape from the vehicle as it was transporting them to jail and that they had also taken a police officer hostage.
The security source said four senior officers had been jailed for four days pending trial but three junior officers had been released.
Heba Morayef, Egypt director at Human Rights Watch, said the detention of the four policemen was an important first step but said more accountability was needed.
"I'm not sure I see this as a turning point. I see this as an exception to the broader impunity enjoyed by security services for the excessive use of force," she said.
"I don't see this as a broader shift because this is such a clear cut case where the state was responsible because these were prisoners and it was impossible to ignore it."
The interim government plans to hold new elections early next year. Foreign powers have called on it to include all political parties and the United States has cut military aid pending progress towards restoring a democratic government through free elections.
The authorities are working on a draft law that would impose severe restrictions on demonstrations.
(Editing by Angus MacSwan)