GENEVA (Reuters) - A regional director of Amnesty International and other senior rights activists detained in Turkey this week are at significant risk of torture, a U.N. human rights spokesman said on Friday.

Amnesty's Idil Eser and the others face a week in detention on suspicion of terrorist group membership after being taken on Wednesday from a training workshop they were holding at a hotel on Buyukada, an island just south of Istanbul.

"We are gravely concerned about all arbitrary arrests and detentions of human rights defenders in Turkey," Elizabeth Throssell, spokeswoman for the U.N. High Commissioner for Human Rights, told a regular U.N. briefing in Geneva.

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"We fear they are now at significant risk of torture and other cruel, inhuman or degrading treatment," she said of the eight Turkish human rights activists and two foreign trainers – a German and a Swedish national - held this week.

Nils Melzer, the U.N. special rapporteur on torture who made an official visit to Turkey last December, had "found evidence of widespread abuse, particularly during initial detention", Throssell said, adding: "So that underscores our fear there."

Turkish authorities have repeatedly denied accusations of torture while saying a firm security stance is needed in the face of dangers it faces from Kurdish militants as well as wars in neighboring Iraq and Syria.

Throssell said the Ankara government seemed to have criminalized the legitimate exercise of rights to freedom of peaceful assembly and freedom of expression under the state of emergency imposed after an attempted coup in July 2016.

The detentions, which Amnesty called a "grotesque abuse of power", came less than a month after a court ordered the arrest of the chairman of Amnesty's Turkey branch on the same charge.

Over the past year, Turkey has jailed more than 50,000 people pending trial and suspended or dismissed some 150,000, including soldiers, police, teachers and public servants, over alleged links with terrorist groups.

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There were concerns regarding their access to lawyers, Throssell said. People on hunger strike are among "nearly 150,000 people arbitrary dismissed" since the coup attempt, she said. "So we're talking big numbers here with regard to human rights defenders and to people who have been sacked."

The purge has alarmed Turkey's Western allies and rights groups, who say the government has used the coup as a pretext to muzzle dissent. Ankara has said the security measures are needed because of the gravity of the threats confronting Turkey.

Separately, the Council of Europe Commissioner for Human Rights, Nils Muiznieks, urged Ankara to immediately release those detained on Thursday and "stop all arbitrary interferences with the legitimate work of civil society".