Israel will target "terror organisations" in Gaza if violence along the territory's border with Israel drags on, the chief military spokesman warned, a day after 15 Palestinians were killed amid protests involving thousands of people near the border fence.
The mass marches were led by Gaza's ruling Hamas group and touted as the launch of a six-week-long protest campaign.
Palestinian health officials said 15 Palestinians were killed by Israeli fire and more than 750 hit by live rounds, making it the bloodiest day in Gaza since the 2014 cross-border war between Israel and Hamas.
During Friday's confrontations, large crowds gathered near the border fence, with smaller groups of protesters rushing forward, throwing stones and burning tyres.
Israeli troops responded with live fire and rubber-coated steel pellets, while drones dropped tear gas from above. The army released video showing soldiers with rifles perched on high embankments overlooking the scene.
Brig Gen Ronen Manelis, the chief army spokesman, denied allegations of excessive use of force, saying those killed by Israeli troops were men between the ages of 18 and 30 who were involved in violence and belonged to militant factions.
He alleged Gaza health officials had exaggerated the number of wounded, and that several dozen at most were injured by live fire while the rest were merely shaken up by tear gas and other riot dispersal means.
Gaza City's Shifa Hospital received 284 injured people on Friday, the majority with bullet injuries, said spokesman Ayman Sahbani. He said 70 were under the age of 18 and 11 were women.
He said 40 surgeries were performed on Friday and that 50 were planned for Saturday. "These are all from live bullets that broke limbs or caused deep, open wounds with damage to nerves and veins," he said.
Protest organisers said mass marches would continue until May 15, the 70th anniversary of Israel's creation. Palestinians mark that date as their "nakba", or catastrophe, when hundreds of thousands were uprooted during the 1948 war over Israel's creation.
The vast majority of Gaza's two million people are descendants of Palestinians who fled or were driven from homes in what is now Israel.
Brig Gen Manelis reiterated that Israel "will not allow a massive breach of the fence into Israeli territory".
He said that Hamas and other Gaza militant groups are using protests as a cover for staging attacks. If violence continues, "we will not be able to continue limiting our activity to the fence area and will act against these terror organisations in other places too", he said.
The border protests were seen as a new attempt by Hamas to break the border blockade, imposed by Israel and Egypt after the Islamic militant group seized Gaza from forces loyal to its rival, Palestinian President Mahmoud Abbas, in 2007. The continued closure has made it increasingly difficult for Hamas to govern.