Nasrin Sotoudeh, a prominent Iranian human rights lawyer, was detained by security forces on Wednesday, according to her family.
Her husband, Reza Khandan, said Sotoudeh was arrested at their home in Tehran and taken to Evin prison, a notorious lockup just outside the city.
"Once, during an interrogation session, I told the interrogators that of all the services that a government must provide for its citizens you only know of one, arresting people," Khandan said in the Facebook post announcing his wife's arrest.
The post did not specify if Sotoudeh had been charged with a crime or why she was detained.
Sotoudeh is a well-known defender of human rights in Iran.
She recently represented a young woman arrested in December 2017 for removing her hijab during a wave of anti-regime protests. Video of the young activist went viral sparking other women to remove their headscarves in public to protest Iran's mandatory veiling laws.
International human rights organizations condemned the arrest and called for her immediate release.
"Nasrin Sotoudeh is a human rights champion who should be applauded, not jailed, for her tireless defense of citizens' rights," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "Iran's judiciary again has revealed to its citizens and the international community its disdain for and fear of people who seek to protect human rights."
Amnesty International called the arrest an "outrageous attack on a brave and prolific human rights defender."
In 2010, Sotoudeh was sentenced to 11 years in prison on charges related to her work defending Iranians detained after the 2009 elections, which acted "against national security." Her sentence was later reduced to six years.
During her imprisonment, a US State Department spokesman called Sotoudeh "a strong voice for rule of law and justice in Iran." She was also awarded the European Parliament's Sakharov Prize for Human Rights in 2012.
She was granted early release in 2013 after serving three years.