"Whoever started the war must end it," Khaled Meshaal told a news conference in Cairo.
Meshaal also said Israeli Prime Minister Benjamin Netanyahu had requested a ceasefire - an assertion the Jewish state immediately denied.
Twelve Palestinian civilians and four fighters were killed in the air strikes, bringing the Gaza death toll since fighting began last Wednesday to 90, more than half of them non-combatants, Gaza officials said. Three Israeli civilians have been killed by Hamas rocket fire over the border.
Meshaal said a truce in Gaza was possible but Hamas would not yield to any Israeli conditions. Hamas did not want an escalation or to draw Israel into a land invasion but was ready to fight back if the conflcit escalated.
Israel had failed to achieve its objectives in Gaza, he said.
"The weapons of the resistance have caught the enemy off guard," he said. "Netanyahu asked for calm and not us."
A senior Israeli government official denied that Netanyahu had requested a truce.
The Hamas leader also called for solidarity in Arab and Palestinian ranks.
"The people of Gaza are not asking for an end of the war, they are asking for their rights, they are asking that Israel end its assassinations and its raids and lift the blockade of Gaza," he said.
Earlier, Israeli missiles blasted a tower block that houses many international media for a second straight day, targeting a computer shop in the building.
Medics said one person died in the attack while several people were wounded. The dead man was believed to be the owner of the shop located on the third floor of the building.
Witnesses said it was hit by at least three missiles, which sent debris flying into the street below and set the shop ablaze.
Israel's military said Sunday's attacks, which also saw a nearby media building hit, were pinpoint strikes on Hamas communication devices located on the buildings' roofs.