He was imprisoned for calling for the resignation of Bahrain's prime minister.
His family said on Monday he began a hunger strike to highlight his detention and ill treatment in prison.
An official at the public prosecutor's office, speaking on condition of anonymity, confirmed Abd Al-Hadi al-Khawaja's imprisonment, but did not elaborate.
Al-Khawaja's wife, Khadija al-Musawi, said on Monday her husband was summoned to a police station on Saturday night and detained.
He was taken to the public prosecutor's office in the capital Manama the same evening where he was informed of the 45-day detention.
Detention without trial
The authorities can hold a suspect for 45 days without charge and the detention order can also be renewed.
The 43-year-old activist was detained a day after he called for the resignation of Prime Minister Shaikh Khalifa bin Salman al-Khalifa, blaming him for economic failures and human rights violations during his more than 30 years in office, his relatives said.
"I am proud of what my husband is doing," said al-Musawi, who said he began a hunger strike when he was detained.
"I know my husband, he won't give up even if he loses his life. I will proudly walk behind his dead body ... and proudly bury him," she told reporters at the Bahrain Centre for Human Rights, where al-Khawaja is the executive director.
Al-Musawi, 46, said her husband did not want to appoint a defence lawyer, because "he does not believe" in the kingdom's justice system.
The Bahrain Centre for Human Rights said it had launched an international campaign to press for his release. "We think his detention is political... We will not rest until [al-Khawaja] is released," said the centre’s president Nabil Rajab.
"We think his detention is political... We will not rest until [al-Khawaja] is released"
president of Bahrain Centre for Human Rights
A small group of pro-democracy activists and relatives demonstrated on Sunday on behalf of al-Khawaja, who returned to Bahrain in 2001 with his family after a 22-year self-imposed exile in Europe.
The Uruba Club, a social and cultural centre where al-Khawaja made his critical comments at a symposium on Friday, was also shut down for 45 days.
Bahrain's king, Shaikh Hamad bin Isa al-Khalifa, warned clubs from holding meetings that defame the country's leaders and damage national unity.
He expressed confidence in the prime minister, who is his uncle, at a Cabinet meeting which the monarch chaired on Sunday, according to the official Bahrain News Agency.
The king has taken bold steps since assuming the throne in 1999 to move the country from an absolute monarchy to a constitutional one.
He has pardoned more than 1000 political prisoners and allowed exiles to return.
In 2002, Bahrain held its first parliamentary elections since 1971.
However, critics charge his reforms do not go far enough towards ensuring freedom of expression and democracy.