Israel unleashed new airstrikes on Gaza early Tuesday, hitting the high-rise home of a Hamas field commander and two border tunnels dug by militants, as Hamas and other armed groups fired dozens of rockets toward Israel.
The escalation in the conflict was sparked by weeks of tensions in contested Jerusalem.
Since sundown Monday when the cross-border attacks began, 24 Palestinians - including nine children - were killed in Gaza, most by airstrikes, Gaza health officials said. The Israeli military said 15 of the dead were militants. During the same period, Gaza militants fired more than 250 rockets toward Israel, injuring six Israeli civilians in a direct hit on an apartment building.
In a further sign of rising tensions, the Israeli army said in a statement that the chief of staff has called in troop reinforcements in the country's south. Israeli Defence Minister Benny Gantz has ordered the mobilisation of 5,000 reserve troops to expand the current campaign "and deepen home front defence".
The exchange of fire Monday night was preceded by hours of clashes between Palestinians and Israeli security forces, including dramatic confrontations at Jerusalem's Al-Aqsa Mosque compound, a sacred site to both Jews and Muslims.
In fighting in the contested city and across the West Bank, more than 700 Palestinians were hurt, including nearly 500 who were treated at hospitals.
In a sign of widening unrest, hundreds of residents of Arab communities across Israel staged overnight demonstrations - denouncing the recent actions of Israeli security forces against Palestinians - in one of the largest protests by Palestinian citizens in Israel in recent years.
The current violence - like previous rounds, including the last intifada, or uprising - was fueled by conflicting claims over Jerusalem. The rival national and religious narratives of Israelis and Palestinians are rooted in the city, making it the emotional core of their long conflict.
In the past, cross-border fighting between Israel and Hamas, the group that rules Gaza, would typically end after a few days, often helped by behind-the-scenes mediation by Qatar, Egypt and others. It was not clear if such a resolution would come this time.
Israeli Prime Minister Benjamin Netanyahu warned Monday that fighting could "continue for some time." Israeli media have reported that the new round of violence is slowing efforts by Netanyahu's rivals to form a ruling coalition among parties with a broad range of ideologies, but a shared goal of toppling Netanyahu.
The support of an Arab-backed party with Islamist roots is key for the anti-Netanyahu bloc's efforts. The party's leader, Mansour Abbas, has essentially said he'll work with whatever political camp offers the most improvements in Arab communities, but the current tensions might deter him from joining a coalition, at least for now.
The current round of violence in Jerusalem coincided with the start of the Muslim fasting month of Ramadan in mid-April. Critics say heavy-handed police measures helped stoke nightly unrest, including a decision to temporarily seal off a popular gathering spot where Palestinian residents would meet after evening prayers. Another flashpoint was the Jerusalem neighbourhood of Sheikh Jarrah, where dozens of Palestinians are under treat of eviction by Jewish settlers.
Over the weekend, confrontations erupted at the Al-Aqsa Mosque compound, which is the third holiest site of Islam and the holiest site in Judaism.
For four successive days, Israel police fired tear gas, stun grenades and rubber bullets at Palestinians in the compound who hurled stones and chairs. Hundreds of Palestinians were hurt, requiring treatment at hospitals. Two dozen officers were also injured. At times, police fired stun grenades into the carpeted mosque.
On Monday evening, Hamas began firing rockets from Gaza, setting off air raid sirens as far as Jerusalem, after giving Israel a deadline to withdraw Israeli security forces from the compound. From there on, the escalation was rapid.
The army said Gaza militants fired more than 250 rockets at Israel with about one-third falling short and landing in Gaza. It said a rocket landed a direct hit on a seven-story apartment block in the coastal Israeli city of Ashkelon.
Photos and videos from the scene showed a large hole in the side of the building. Israeli paramedic service Magen David Adom said it treated six people injured in the rocket strike. Two were hospitalised in moderate condition.
In Gaza, most of the deaths were attributed to airstrikes. However, seven of the deaths were members of a single family, including three children, who died in an explosion in the northern Gaza town of Beit Hanoun and it was not clear if the blast was caused by an Israeli airstrike or errant rocket.
More than 100 Gazans were wounded in the airstrikes, the Health Ministry said.
Israel's tactics in Jerusalem have drawn angry reactions from the Muslim world.
Regional power house Saudi Arabia said in a statement that it condemns in the strongest terms what it said were attacks by Israeli forces against the sanctity of Al-Aqsa and the safety of its worshippers. The Saudi Foreign Ministry called Tuesday on the international community to hold Israeli forces responsible for any escalation.
Separately, the 57-nation Organisation of Islamic Cooperation is holding an emergency meeting of its permanent representatives in Jiddah to discuss the tensions.