* Dozens of Islamists died in incident last year
* Different explanations given for their deaths
* Government cracking down on Muslim Brotherhood (Adds Islamist press conference, activist)
By Michael Georgy
CAIRO, March 18 (Reuters) - A Cairo court sentenced a police officer to 10 years in prison with labour on Tuesday in connection with the deaths of 37 Islamists last year, judicial sources said, one of the most controversial incidents since an army takeover last July.
Three other policemen were given one-year suspended sentences, they said.
The Interior Ministry said at the time that the Islamists, Muslim Brotherhood members and their supporters, had died during an attempted prison break after being suffocated by tear gas in the incident last August.
However, a legal source said the men had died from asphyxiation in the back of a crammed police van while they were being moved to a jail on the outskirts of Cairo.
The government has launched a widespread crackdown on the Brotherhood since the army toppled Islamist president Mohamed Mursi last July after mass protests against his rule.
The Islamist movement has accused the authorities of large-scale human rights abuses. The government has denied the allegations and declared the Brotherhood a terrorist group which poses a grave security threat to the most populous Arab nation.
Islamists loyal to Mursi said on Tuesday that Egyptian security forces prevented them holding a news conference about the deaths of hundreds of Brotherhood members last year.
A pro-Mursi alliance said on its Facebook page that police stormed a building where they were due to respond to a government-appointed panel on the deaths at a Cairo protest camp. Reuters witnesses said police vans surounded the area.
An Interior Ministry official said authorities had no immediate comment on the incident. A security source said two people were briefly detained on suspicion of membership in the outlawed Brotherood.
The panel said this month that the deaths of hundreds of Brotherhood supporters at a protest camp in Cairo last August was mostly the fault of demonstrators who had provoked the security forces into opening fire.
The findings mainly echoed the military-backed government's version of events. But in an unexpected move, the panel also placed some responsibility for the bloodshed on the security forces and said they had used disproptionate force.
Human rights activists accuse the judiciary of double standards, imposing stiff penalties on Islamists while taking a softer view of crimes committed by security forces.
The court sentenced Lieutenant Colonel Amr Farouk, deputy head of Heliopolis police station, to 10 years in jail with labour and three other policemen to one year suspended sentences on charges of involuntary manslaughter and extreme negligence.
"We are suffering from a lopsided judicial system that reverted to the most lenient punishment because the accused person was a policeman," said lawyer and human rights activist Gamal Eid.
"I see the crime as premeditated murder and not involunatry manslaughter."
Security forces have killed hundreds of Muslim Brotherhood members in the streets, arrested thousands of others and put top leaders on trial since army chief Field Marshal Abdel Fattah al-Sisi deposed the country's first freely-elected government.
The Brotherhood, which has an estimated 800,000 members, has gone underground but is unlikely to go away after surviving repression under one Egyptian autocrat after another.
Although neither side is showing flexibility just now, political analysts say reconciliation may be the only way to bring stability to Egypt, which has a peace treaty with Israel and controls the Suez Canal, a vital global shipping lane.
The Interior Ministry, which was dreaded under the rule of president Hosni Mubarak, has put itself squarely back in the centre of power after a period of relative uncertainty after he was toppled in an army-backed popular uprising in 2011. (Editing by Ralph Boulton)