http://www.bbc.co.uk/news/world-middle-east-13373006
Israeli forces have fired on groups of protesters at borders with the Palestinian territories, Syria and Lebanon.
Unconfirmed reports say several people have died, and dozens have been injured.
In one incident, thousands of Palestinian supporters from Syria entered the Golan Heights, Israel says.
Palestinians are marking the Nakba or Catastrophe, their term for the founding of the Israeli state in 1948.
Hundreds of thousands of Palestinians fled or were forced out of their homes in fighting after its creation.
Impetus
Clashes have been taking place at four separate border crossing points - in Gaza, the West Bank, the Golan Heights and the frontier with Lebanon.
The BBC's Jon Donnison, in the West Bank town of Ramallah, said this year's Nakba protests have been given impetus by the uprisings in countries across the Middle East and North Africa.
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We are seeing here an Iranian provocation, on both the Syrian and the Lebanese frontiers, to try to exploit the Nakba day commemorations
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Brig Gen Yoav Mordechai
Israeli army
In pictures: Palestinian rallies
Our correspondent, at the Qalandiya checkpoint there, says there is a stand-off now, but dozens of Palestinians have been injured.
Palestinian protesters have been throwing stones at Israeli security forces, who have been firing tear gas and rubber bullets.
On the occupied Golan Heights, the Israeli military said it had only fired warning shots as a large number of protesters tried to breach a border fence near the village of Majdal Shams.
But unconfirmed reports said four people had been killed and at least 10 people injured.
Israel's army says this is a "serious" incursion. Brig Gen Yoav Mordechai said soldiers were still trying to control the crowds and that dozens of protesters had crossed.
The army has reportedly sealed off Majdal Shams and is carrying out house-to-house searches for infiltrators.
Israeli seized the strategic territory from Syria in 1967.
On the Lebanon-Israel border, a large number of protesters also approached the crossing with Israel.
Dozens of buses had brought protesters to the area under the rally slogan of "March for the return to Palestine".
On the Israel-Gaza frontier, at the Erez border crossing, Israeli troops opened fire with tanks and machine guns, injuring dozens, Palestinian medical officials said.