The brief detention caused a swift furore from other rights activists, including Amnesty International, which put out a statement demanding the release of Ismail, whose work on women's rights has been recognised internationally.
She was released later Friday, Shahjahan said - but officials kept her travel documents.
"I kept on waiting just to make sure that she is not disappeared," Shahjahan added.
Earlier, Ismail had released an audio message on her detention which was verified by AFP.
She said she had been detained for speaking at a rally in August for the Pashtun Protection Movement (PTM), a group calling for rights for the Pashtun tribal population of Pakistan's restive northwest.
Nine other people detained under the same orders were still being held at a police station in Swabi, a district of northwestern Khyber Pakhtunkhwa (KP) province where the rally took place, Shahjahan told AFP.
The PTM emerged at the start of this year with marches demanding the end of extra-judicial executions and enforced disappearances committed by security forces against Pashtuns living in KP and tribal areas along the border with Afghanistan.
"Support for @Gulalai_Ismail by activists and general public proved fruitful," the group tweeted after Ismail's release.
A photo of the activist smiling and raising her first while surrounded by six other women, including Shahjahan, swiftly did the rounds on social media.
The image was sent to AFP by a PTM activist. Ismail's detention comes as Pakistan's media and civil society have complained of ever-increasing pressure from the powerful army.
"This is not an attack on Gulalai Ismail or PTM," the activist said in her audio message. "This is an attack on our freedom of speech." Ismail was still a teenager in 2002 when she co-founded Aware Girls, an NGO that promotes gender equality in deeply conservative KP.
Among other accolades, she was awarded the Chirac Foundation Conflict Prevention Prize in 2016 and the Anna Politkovskaya Award in 2017.
"Instead of trying to silence human rights defenders, the new government must create a secure framework for those who raise their voices in the name of justice," said Rabia Mehmood, a researcher for Amnesty, during Ismail's detention.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)