The head of a hardline Islamabad mosque Friday called an army offensive against the Taliban "un-Islamic" and said the militants' massacre of 133 children was in retaliation for air strikes against them.

The Tehreek-e-Taliban Pakistan (TTP), claimed the assault as revenge for an ongoing military offensive against its strongholds in the tribal northwest.

Maulana Abdul Aziz told worshippers at the Red Mosque he "shared the grief" of the victims' families but said the TTP's response was understandable.

"O rulers, O people in power, if you will commit such acts, there will be a reaction," he said.

The army has killed more than 1,700 suspected militants since June in operations against bases of TTP and other militants in North Waziristan and Khyber tribal areas.

In the wake of the Peshawar attack the army has pledged to redouble its efforts to wipe out the scourge of militancy, but Aziz condemned their struggle.

"This operation in the North Waziristan is un-Islamic," he said.

"You may debate it, you may call scholars from abroad, from India and Bangladesh, I will go before them and prove it that this operation is un-Islamic."

The Red Mosque, which stands a stone's throw from the parliament buildings in the centre of the capital, was the scene in 2007 of a week-long military siege against radicals which left more than 100 people dead.

The scale of the Peshawar attack and the fact the victims were almost all children has horrified Pakistan and prompted many to demand action against Taliban sympathisers, including radical preachers.

But a civil society protest outside the Red Mosque on Friday afternoon was broken up by police, while members of a hardline group were allowed to stage their own demonstration nearby.

Earlier this month female students affiliated with the Red Mosque issued a video statement praising the Islamic State group and calling on it to avenge the death of Osama bin Laden.

The women belong to the Jamia Hafsa seminary which in April named its library in honour of the slain Al-Qaeda leader.