Ismail was arrested along with 17 other members of the PTM during a protest in Islamabad on Tuesday.
"It was very painful for my family, who took 30 hours to know where I was," she said, adding that she had been freed late Wednesday.
She was separated from the other activists, she said, and was not placed under official arrest.
"They didn't charge me with anything. They didn't let me contact my lawyer. They just kept moving me around," she said.
An Islamabad Police official dealing with the case swept the claims aside.
Police follow "a proper procedure and her claims of an encounter are unthinkable," he said.
He accused Ismail and the other activists of uttering "anti-state slogans".
Ismail is also the co-founder of Aware Girls, an NGO that promotes gender equality. She has received several international awards.
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