Ismail Mashal, an outspoken critic of the Taliban's ban on education for women and girls, was detained on Thursday.
"(I am) concerned about yesterday's arrest of peaceful education activist and university lecturer Ismael Mashal by the Taliban," the UN's Bennett wrote on Twitter.
He called for Mashal's immediate and unconditional release.
Mashal's aide Farid Ahmad Fazli told the AFP news agency, he was "mercilessly beaten and taken away. "He is still in detention, and we don't know where he is being held."
Taliban confirms Mashal's detention
Mashal was detained while handing out free books in Kabul.
The Taliban, however, said he was gathering journalists, creating a crowd on the street and "creating propaganda against the government".
"Teacher Mashal had indulged in provocative actions against the system for some time," tweeted Abdul Haq Hammad, director at the Taliban's Ministry of Information and Culture.
"The security agencies took him for investigation."
Hammad, later said, had visited Mashal in detention and found he was being held in good conditions.
It wasn't immediately clear if he would be formally charged.
Taliban's restrictions on women
In December, Mashal tore up his academic records on television to protest against the Taliban's ban oneducation for women and girls.
Mashal, who had been a lecturer for more than a decade at three Kabul universities, said he would stand up for women's rights.
"As a man and as a teacher, I was unable to do anything else for them, and I felt that my certificates had become useless. So, I tore them," he said at the time.
"I'm raising my voice. I'm standing with my sisters... My protest will continue even if it costs my life," Mashal added.
Women have been been barred from going to parks, gyms and public baths and also prevented from working with local and international NGO's.
The international community has condemned Taliban restrictions on women.
lo/jcg (AFP, Reuters)