Describing the assassination of Ismail Haniyeh as "terrorism", Islamabad's foreign ministry said in a statement that "Pakistan views with serious concern the growing Israeli adventurism in the region."
"Its latest acts constitute a dangerous escalation in an already volatile region and undermine efforts for peace", it said.
In the evening, more than a thousand supporters of the country's largest Islamist party, Jamaat-e-Islami, gathered in Rawalpindi, a large city on the outskirts of Islamabad, hailing the "martyr Ismail Haniyeh" while booing Israel and America.
"I am here to show solidarity with my brothers and sisters in Palestine", Syed Obaid ur Rehman, a 25-year-old student, told AFP.
"I cannot be there physically but at least I can demonstrate here to make the voice of Palestine heard."
Sajid Hussain, a 28-year-old civil servant, told AFP that Haniyeh's "martyrdom is a huge loss for the Muslim world".
Haniyeh had travelled to Tehran to attend Tuesday's swearing-in of Iran's new President Masoud Pezeshkian.
Iran is a sworn enemy of Israel and ally of Hamas and Lebanon's Hezbollah.
It and Hamas have vowed to avenge Haniyeh's death, raising fears of a flare-up in the region amid the war in Gaza.
Israel -- which Pakistan does not recognize -- declined to comment on the death of Haniyeh, 61, who was elected head of Hamas's political bureau in 2017.
His death came just hours after Israel attacked a Hezbollah stronghold in south Beirut on Tuesday, killing a senior commander of the Lebanese militant group it blamed for a deadly weekend rocket strike on the Israeli-annexed Golan Heights.
