Israeli security forces are reported to have killed at least five people following clashes with Palestinian protesters on the border with Gaza.
The Palestinian health ministry said at least 500 Palestinians were hurt by live fire, rubber-coated steel pellets or tear gas fired by Israeli forces at several locations along the fence, but did not provide the breakdown.
Earlier on Friday, in a separate incident, a Palestinian farmer was killed by an Israeli tank shell while he was working in his field before dawn.
The protests had begun as mass sit-ins organised by Gaza's Hamas rulers, but quickly spun out of control.
Israel's military said thousands of Palestinians rolled burning tyres and threw stones at forces stationed on the border, and that troops opened fire at the "main instigators".
Palestinian witnesses said hundreds of Palestinians participated in the clashes, while thousands more gathered in tent encampments set up in five sites near the border.
Such mass gatherings near the border signal a new tactic by Hamas – and one that might prove more challenging to Israel's military than previous smaller protests.
Military officials have said they will respond harshly to any breaches of the border fence. At the same time, a rising number of casualties will likely stoke more border tensions, a scenario Israel hopes to avoid.
The sit-ins are seen as a new attempt by Hamas to break a crippling, decade-old Gaza border blockade by Israel and Egypt that has made it increasingly difficult for the Islamic militant group to govern.
Other tactics over the years, including Hamas' cross-border wars with Israel and attempts to reconcile with political rival Mahmoud Abbas, the West Bank-based Palestinian president, have failed to end Gaza's isolation.
Hamas spokesman Fawzi Barhoum praised the turnout.
"The large crowds reflect the Palestinian people's determination to achieve the right of return and break the siege and no force can stop this right," he said.
Friday's actions are to be the first in a series of protests planned in Gaza in coming weeks. The protests are to culminate on May 15, the 70th anniversary of Israel's creation, with a march through the border fence.
Palestinians commemorate the date as the anniversary of their mass displacement and uprooting during the 1948 war over Israel's creation. The vast majority of Gaza residents are descendants of Palestinians who fled or were driven from communities in what is now Israel.
Israel's military said ahead of Friday's protests that it doubled its standard troop level along the border, deploying snipers, special forces and paramilitary border police units, which specialise in riot control.
Maj Gen Eyal Zamir, commander of the Israeli military's Southern Command, which includes the border, said Friday that "we are identifying attempts to carry out terror attacks under the camouflage of riots".
He urged Gaza residents to stay away from the border, and held Hamas responsible for any violence there.
Previous protests near the border fence in recent months have turned deadly, with Israeli soldiers firing live bullets at Palestinians burning tyres, throwing stones or hurling firebombs.
On Friday, mosques across Gaza called on Palestinians to join the protests. Buses took protesters to the border area.
Several hours before the confrontations, a Palestinian farmer identified as 27-year-old Amr Samour was killed by an Israeli tank shell in southern Gaza, the health ministry said.
Israel said troops had directed tank fire at suspicious figures near the border fence in the area.
Yasser Samour, a relative and fellow farmer, said Amr Samour was harvesting parsley before dawn, in hopes of selling it fresh in the market later in the day.
"I was working on the next field,"
Mr Samour said. "We heard shelling landing on the field where Amr works. We ran there and found him hit directly with a shell. We were more than a kilometre away from the border."
Another farmer was wounded in the leg by shrapnel, Mr Samour said.
