Turkish police rounded up eight leading human rights campaigners, including the head of rights group Amnesty International in Turkey, Idil Eser, at a meeting on an island near Istanbul, media said on Thursday.

They had gathered at a hotel on Buyukada island, just south of Turkey's largest city, when they were taken to a police station on Wednesday, the Hurriyet newspaper said. It was not clear why they were held.

Amnesty called for the group's immediate release, saying it was "profoundly disturbed and outraged" at the police actions during a digital security workshop.

This is a grotesque abuse of power and highlights the precarious situation facing human rights activists in the country - Salil Shetty, Amnesty International

Turkish police were holding 12 people in total, Reuters reported. Among those detained were Eser and seven other human rights activists, two foreign trainers – a German and a Swedish national – as well as the hotel owner, Amnesty's statement said.

"This is a grotesque abuse of power and highlights the precarious situation facing human rights activists in the country," said Amnesty's secretary general Salil Shetty.

“We are profoundly disturbed and outraged that some of Turkey’s leading human rights defenders, including the director of Amnesty International Turkey, should have been detained so blatantly without cause.

“Her incommunicado detention and that of the other human rights defenders attending a routine training event, is a grotesque abuse of power and highlights the precarious situation facing human rights activists in the country."

The rights campaigners said that the Amnesty staff must be "immediately and unconditionally released". The whereabouts of those held were unknown.

The seven rights activists arrested with Eser on Wednesday evening were named as: Ilknur Ustun of Women's Coalition; Gunal Kursun and Veli Acu of Human Rights Agenda Association; Nalan Erkem and Ozlem Dalkiran of Citizens Assembly; Nejat Tastan of Equal Rights Watch Association and lawyer Seyhmus Ozbekli, Amnesty said.

Amnesty International's Turkey chair, Taner Kilic, is also in police custody. He was arrested on 6 June with 22 other lawyers and charged with membership of a "terrorist" organisation. Amnesty called the charges "baseless".

Amnesty staff said the detentions on Wednesday came less than a month after campaign group's chairman in Turkey, Taner Kiliç, was remanded in custody on what the group described as “baseless charges” of links to Fethullah Gülen, who is accused of masterminding the failed coup on 15 July 2016.

So the chair and now the director of Amnesty International Turkey are detained - Erdogan is taking his crackdown into much darker places pic.twitter.com/d5kmPDjHKS — kristyan benedict (@KreaseChan) July 6, 2017

The EU's enlargement commissioner, Johannes Hahn, said the arrests would form part of his discussions in Ankara this week.

"The arrest of people, for instance last night's arrests, we have to address these issues in friendship and mutual understanding. That's why we are here," he said.

In a security crackdown following the attempted coup, Turkey has jailed more than 50,000 people pending trial, with 150,000 suspended or dismissed from their jobs.

More than 240 people were killed in the failed putsch and the government has said the security measures are necessary given the gravity of the threats facing Turkey. Gülen denies any involvement.

The subsequent crackdown initially focused on people with suspected ties to the coup attempt, but has been extended to include other government opponents, including activists and politicians.