Rights defenders are often accused of treasonous activity in Turkey, in an attempt to discredit their work. One article on a pro-government news site accused those arrested on Wednesday of being spies.
The arrests are the latest salvo in a crackdown on opposition politicians, journalists and activists in Turkey that has seen an estimated 50,000 people arrested and more than 140,000 dismissed or suspended from their jobs.
The government of President Recep Tayyip Erdogan says the purge is necessary to round up the coup plotters, but critics say the coup is being used as a smoke screen to target legitimate opposition.
Thousands of academics have been dismissed under the crackdown, while more than a dozen lawmakers are in jail, as are more than 170 journalists, though exact numbers vary.
Tens of thousands of Turks are marching to Istanbul, Turkey’s largest city, from Ankara, the capital, to protest a perceived breakdown in the country’s judicial process.
But Mr. Erdogan still has significant support: Just over half of the electorate voted to grant his office more power in a recent referendum, in a poll marred by ballot irregularities.
The secretary general of Amnesty, Salih Shetty, linked the detention of Ms. Eser to the wider state of human rights in Turkey.
“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,” Mr. Shetty said in a statement.
He added: “This is a grotesque abuse of power and highlights the precarious situation facing human rights activists in the country.”