Rehman had been a key adviser to Asif Ali Zardari, Pakistan's president, and her resignation will be a blow to his embattled government.
"She is one of the people that many say managed to manoeuvre Ali Asif Zardari into power," Khan reported.
Rehman's resignation is reportedly linked to the government's clampdown on the country's media as opposition workers and lawyers undertake a "long march" towards the capital, Islamabad.
Pakistan's Geo News TV channel claimed Zardari had ordered it be blocked across the country, the Press Trust of India reported.
Zardari's office dismissed the claim.
Zardari's government has tried hard to suppress anti-government protesters marching on the capital, arresting about 100 demonstrators and preventing another 300 from leaving Karachi, where the march began on Thursday.
Pakistan's crisis deepened in February when the supreme court banned Nawaz Sharif, leader of the main opposition PML-N party, and his brother Shahbaz from elected office.
Hours after the court's verdict against Sharif, Zardari dismissed the Punjab provincial administration led by Sharif's brother and handed the reins to a federally appointed regional governor.
The so-called "long march" was to demand the reinstatement of judges sacked by Pervez Musharraf, the former president, and had the support of Sharif, putting the country's two largest political parties on a collision course.
Despite the crackdown, lawyers, opposition parties and civil activists have vowed to intensify their efforts against the government and late on Friday reports suggested the government was considering a deal with the opposition.
The Associated Press reported a senior presidential aide as saying Zardari was considering letting the opposition regain the leadership of the Punjab, the country's most powerful province.
A second presidential aide said "it looks like we are going toward some kind of resolution" to the crisis, but cautioned that the details of any deal were still being worked out.
A spokesman for the opposition declined to discuss any possible agreement.
Zardari's office said in a statement on Saturday that the government wanted to defuse tension through dialogue and to restore "political normalcy", but it also said that the authorities would maintain law and order "at all cost".