Israeli troops wounded at least 65 Palestinians at the Gaza border during protests to mark the 71st anniversary of the "Nakba", or catastrophe, when many Palestinians lost their homes in the fighting around Israel's creation, Gaza officials said.
Thousands had gathered at the frontier with Israel, the scene of bloodshed over the past year that has raised international concern.
Groups approached the border fence, planting Palestinian flags and throwing stones towards Israeli soldiers on the other side despite the efforts of marshals to keep protesters away from the barrier, witnesses said.
Israeli troops fired tear gas, rubber bullets and live ammunition, the witnesses said.
It is not clear how many of those who were injured were hit by live ammunition or were hurt by rubber bullets or by inhaling tear gas.
The Israeli military said that about 10,000 rioters and demonstrators gathered in several places along the Gaza border.
It said tyres were set on fire and explosive devices and rocks were thrown as well as a number of attempts to "approach the security fence. IDF troops are responding with riot dispersal means," the Israeli Defence Forces said.
The rallies were called to mark Nakba Day, what Palestinians term the catastrophe that befell them at Israel's creation in 1948, when hundreds of thousands fled or were expelled from lands in what is now Israel.
Palestinians also held rallies in the occupied West Bank but no major clashes with Israeli forces were immediately reported.
This year's Nakba protests were preceded by a surge in deadly cross-border fighting between Gaza militants and Israel which ended in a ceasefire on 6 May.
Israeli troops have killed more than 200 Palestinians and wounded thousands in regular border protests since March 2018, according to human rights groups.
United Nations investigators have said the Israeli military might be guilty of war crimes for using excessive force.
Israel has said it is defending its border against attacks against its troops and infiltration attempts by gunmen.
Israel has rejected a Palestinian right of return as a threat to maintaining a Jewish majority in a country it describes as the nation-state of the Jewish people.
Frustration is growing among Palestinians as hopes fade for a two-state solution to the conflict which would give them an independent country.
US President Donald Trump's announcement in December 2017 of US recognition of disputed Jerusalem as Israel's capital also fuelled Palestinian anger.