Waqar Kiani, who once worked for Britain's Guardian newspaper, said he was driving home last night when his car was stopped by four men in police uniforms in Sector G-8.
The men, who told him that they wanted to check his car, pulled Kiani out of his vehicle and beat him up.
He was later admitted to the Pakistan Institute of Medical Sciences with severe injuries to the back and face. "I have injuries in the back and on my face. They had come in a police van," Kiani told Dawn newspaper from hospital bed.
Interior Minister Rehman Malik directed police to probe the incident and submit a report. Acting Islamabad police chief Bin Yamin said no policemen were involved in the incident.
Journalists and rights groups condemned the incident, which occurred about three weeks after journalist Syed Saleem Shahzad was abducted and killed while driving from his home to a TV station in Islamabad on May 29.
Shahzad was murdered just two days after he alleged in an article that al-Qaeda had infiltrated the Pakistan navy.
Referring to the assault on Kiani, Ali Dayan Hasan, the Pakistan representative of Human Rights Watch, said in a message posted on Twitter: "Continuing attacks on journalists in Pakistan are reprehensible and unacceptable."
In an article published in the Guardian on June 13, Kiani had recounted his abduction and torture by suspected intelligence operatives in July 2008.
He told the newspaper that his car was forced off the road by two other vehicles and he was then taken to a safe house and tortured by men who accused him of being a "British agent."
Kiani was later dumped at Mianwali in Punjab. He continued to receive threats after his abduction.
His harassment stopped after then British Foreign Secretary David Miliband wrote a personal letter to the Pakistan government, expressing concern.