By Malcolm Brabant
BBC News, Athens
Refugees signed witness statements saying police beat the teenager
The UN refugee agency is demanding an inquiry into alleged police brutality at a notorious detention centre on the Greek island of Lesbos.
Several asylum seekers were reportedly beaten after protesting over prolonged detention and the Pagani centre's cramped and insanitary conditions.
The new socialist government in Greece recently announced a zero tolerance policy towards police violence.
Human rights workers on Lesbos say a 17-year-old Kurd was seriously hurt.
They claim four police guards beat the asylum seeker for half an hour until he became unconscious and was taken to hospital.
A UN expert on torture, who has seen pictures of the alleged abuse on the young Kurd, said: "The bruising... looks fairly typical of what are known as 'tramline' bruises that result from a beating with a round or square section of rod or stick.
"Of course, one cannot be certain just from these photographs that a beating was the actual cause of the bruising, but it does look fairly typical," he added.
Several other migrants were allegedly slapped and punched as they left their cells while escaping a fire started in the centre by those protesting against the conditions.
Charity workers on Lesbos claim the beating of the teenager took place in front of 40 other refugees, who have signed affidavits.
The workers said the police were trying to intimidate witnesses into staying silent.
The UN refugee agency, the UNHCR, has called on the Greek government to close the centre.
Such a place should not exist in Europe, according to one senior UN official.
Pagani was designed to accommodate 200, but so many people are being trafficked from just across the water in Turkey, that it sometimes houses 1,200 migrants.
For example, one cell containing 200 women and children has only two toilets, which have overflowed and soiled mattresses.
The smell is unbearable, says the UNHCR.
Pagani was visited last week by Greece's new deputy Citizen's Protection Minister, Spyros Vougias, who admitted the conditions were inhuman.
His ministry has promised to build new centres, and also to reform elements within the Greek police by eradicating institutionalised violence.
Greece's reluctance to grant asylum to genuine refugees has earned it a bad reputation throughout Europe.
The new government has pledged to improve human rights, but at the same time is asking for European help to protect its long coastal border from infiltration by illegal immigrants.