Some youths hurled rocks and petrol bombs at Israeli troops. Others used wire-cutters to try to breach the border fence. Though witnesses say most protesters were unarmed, army footage aired on Israel's Hadashot News showed a group with with guns, bombs, grenades and wire-cutters trying to cross the fence; all eight were shot dead.
Ambulances and hospitals were overwhelmed, said Ayman Sahabani, chief of surgery at Shifa Hospital in Gaza City. "We're at 25 times our capacity and our drug inventory is exhausted," he said. Some reports put the number of hospitalised Palestinians at more than 1300.
Israeli army spokesman Lieutenant Colonel Jonathan Conricus said troops tried using non-lethal weapons like tear gas and rubber bullets when possible and had tried dousing burning tires with water cannons, but those methods had not proven sufficiently effective.
"We are defending ourselves against terrorists and rioters," he said. "We shoot in order to stop them so that they cannot harm the security infrastructure and infiltrate into Israel."
Ranked as the world's seventh-largest arms exporter, Israel has developed expertise in making non-lethal weapons to subdue protests in Gaza and the West Bank. Among them are putrid-smelling fluids and immobilising foam. In preparation for this week's demonstrations, it equipped drones to drop tear gas.
Given the availability of those methods, UN special coordinator for the Middle East peace process Nickolay Mladenov told the Security Council on Tuesday there was no justification for the killings.
"Israel has a responsibility to calibrate its use of force, to not use lethal force, except as a last resort, under imminent threat of death or serious injury," he said. "It must protect its borders, but it must do so proportionally and investigate every incident that has led to a loss of human life."
Israel faced criticism from countries including Saudi Arabia, France, Belgium and the UK Turkey expelled Israel's ambassador and recalled its own from Israel and the US. The Palestinian Authority appealed to the Arab League and UN for international protection and urged the International Criminal Court to investigate Israeli officials.
Israel is "conducting a massacre against our people," said Mahmoud al-Aloul, deputy chairman of Palestinian Authority President Mahmoud Abbas's ruling Fatah party.
In the 2008 Gaza conflict that left about 1400 Palestinians and five Israelis dead, a UN fact-finding commission found both Israel and Hamas, the group which rules Gaza, committed war crimes. The panel's chief, Richard Goldstone, later said the report erroneously accused Israel of intentionally killing civilians.
Hamas said 50 of the Monday's dead were members of the armed Islamist group, which is designated a terrorist organisation by the US and European Union.
Following investigations in the aftermath of three Gaza wars, the Israeli army has assigned lawyers, senior officers and videographers to its regional commands to document military actions and ensure no war crimes are committed. The army says its soldiers adhere to open-fire regulations and officers were prepared to defend their actions.
"Every bullet that's fired is ordered by a commander and every bullet has a target," said Conricus. Still, he said, smoke, sand, wind and tear gas created a chaotic environment in Gaza, and "unfortunately we do not always hit where we are trying to hit."
Israeli gunfire "will never make us lose hope to get back our land," said Mohamed Abu el-Majd, 23, whose left leg was fractured. "They can shoot me in the right leg now and I swear to God I'll come crawling back."