Khalid Khawaja, who heads the Defence of Human Rights group, has said he was detained on the orders of Prime Minister Shuakat Aziz. "After I was abducted, they kept moving me around so that my family could not trace me," Khawaja told the BBC News website.
"From the time I was kidnapped, nobody was able to tell me for what reason I was detained. He said he was abducted on his way to prayers in Islamabad. He was released without charge on Monday. Khawaja wrote a letter from his cell describing his abduction.
A Pakistan human rights activist says that he will prosecute members of the security services who abducted him in January and kept him under detention. His rights group says it champions those who have been illegally detained by the intelligence agencies.
Critics say that Khawaja - a retired air force officer who has served in senior position in Pakistan's powerful Inter Services Intelligence (ISI) agency - has close links to the Taliban and al Qaeda.
His disappearance received widespread coverage, and was taken up by Chief Justice Iftikhar Chaudhry who was later suspended by President Pervez Musharraf. Hundreds of Pakistanis have disappeared since 2001, according to Human Rights Commission of Pakistan. "They eventually told me I was being kept on the orders of the prime minister," he said.
He said the men who abducted him were in plain clothes, and he is unsure about their identity. Khawaja says he was held in at least five different jails so that his whereabouts could be kept uncertain.
"They wanted to make me disappear, because the cause of the missing people had touched a raw nerve," he said. Eventually, after much pressure from the courts, he was released. Activists say abductions by the security agencies have become routine.
Following news of his abduction, Human Rights Watch described his disappearance as "suspicious" and urged the authorities to reveal his whereabouts. The next day, Khawaja was brought before a court in Islamabad and remanded in custody for three days. He denied a charges of distributing banned literature.