Several people have been killed and scores of others wounded in the Gaza Strip, Golan Heights, Ras Maroun in Lebanon and the Israeli-occupied West Bank, as Palestinians mark the "Nakba", or day of "catastrophe".
The "Nakba" is how Palestinians refer to the 1948 founding of the state of Israel, when an estimated 700,000 Palestinians fled or were expelled following Israel's declaration of statehood.
At least one Palestinian was killed and up to 80 others wounded in northern Gaza as Israeli troops opened fire on a march of at least 1,000 people heading towards the Erez crossing between the Gaza Strip and Israel.
A group of Palestinians, including children, marching to mark the "Nakba" were shot by the Israeli army after crossing a Hamas checkpoint and entering what Israel calls a "buffer zone" - an empty area between checkpoints where Israeli soldiers generally shoot trespassers, Al Jazeera's Nicole Johnston reported from Gaza City on Sunday.
"We are just hearing that one person has been killed and about 80 people have been injured," Johnston said.
"There are about 500-600 Palestinian youth gathered at the Erez border crossing point. They don't usually march as far as the border. There has been intermittent gunfire from the Israeli side for the last couple of hours.
"Hamas has asked us to leave; they are trying to move people away from the Israeli border. They say seeing so many people at the border indicates a shift in politics in the area."
Separately in south Tel Aviv, one Israeli man was killed and 17 were injured when a 22-year-old Arab Israeli driver drove his truck into a number of vehicles on one of the city's main roads.
Israeli police spokesman Micky Rosenfeld said the driver, from an Arab village called Kfar Qasim in the West Bank, was arrested at the scene and is being questioned.
"Based on the destruction and the damage at the scene, we have reason to believe that it was carried out deliberately," Rosenfeld said. But he said he did not believe the motive was directly linked to the anniversary of the Nakba.
West Bank clashes
One of the biggest Nakba demonstrations was held near Qalandiya refugee camp and checkpoint, the main secured entry point into the West Bank from Israel, where about 100 protesters marched, Al Jazeera's Nisreen El-Shamayleh reported from Ramallah.
Some injuries were reported from tear gas canisters fired at protesters there, El-Shamayleh said.
Small clashes were reported throughout various neighbourhoods of East Jerusalem and cities in the West Bank, between stone-throwing Palestinians and Israeli security forces.
Israeli police said 20 arrests were made in the East Jerusalem area of Issawiyah for throwing stones and petrol bombs at Israeli border police officers.
About 70 arrests have been made in East Jerusalem throughout the Nakba protests that began on Friday, two days ahead of the May 15 anniversary, police spokesman Rosenfeld said.
Tensions had risen a day earlier after a 17-year-old Palestinian boy died of a gunshot wound suffered amid clashes on Friday in Silwan, another East Jerusalem neighbourhood.
Police said the source of the gunfire was unclear and that police were investigating, while local sources told Al Jazeera that the teen was shot in random firing of live ammunition by guards of Jewish settlers living in nearby Beit Yonatan.
Meanwhile, Syrian state television reported that Israeli forces killed four Syrian citizens who had been taking part in an anti-Israeli rally on the Syrian side of the Israeli-occupied Golan Heights border on Sunday.
Israeli army radio said earlier that dozens were wounded when Palestinian refugees from the Syrian side of the Golan Heights border were shot for trying to break through the frontier fence. There was no comment on reports of the injured.
There have also been reports that Israeli gunfire killed up to 10 people and injured scores more in the Lebanese town of Ras Maroun, on the southern border with Israel.
Matthew Cassel, a journalist in the town, told Al Jazeera that he saw at least two dead Palestinian refugees.
"Tens of thousands of refugees marched to the border fence to demand their right to return where they were met by Israeli soldiers," he said.
"Many were killed. I don't know how many but I saw with my own eyes a number of unconscious and injured, and at least two dead.
"Now the Lebanese army has moved in, people are running back up the mountain to get away from the army."
'End to Zionist project'
Israeli prime minister Binyamin Netanyahu addressed the events of "Nakba Day" in a televised statement on Sunday, particualrly referring to attempts to infiltrate Israel's borders with Syria, Lebanon and the Gaza Strip, saying "we are determined to defend our borders".
Netanyahu said that he instructed Israeli forces to act with restraint, but to stop all attempts at infiltration and challenges to Israel's sovereignty.
He said that the "Nakba Day" protesters were not fighting for the 1967 borders as they claim, but were denying Israel's right to exist.
"We must understand who and what we are up against," Netanyahu said.
Earlier on Sunday Ismail Haniyeh, prime minister of Hamas-controlled Gaza, repeated the group's call for the end of the state of Israel.
Addressing Muslim worshippers in Gaza City on Sunday, Haniyeh said Palestinians marked this year's "Nakba" "with great hope of bringing to an end the Zionist project in Palestine".
"To achieve our goals in the liberation of our occupied land, we should have one leadership,'' Haniyeh
said, praising the recent unity deal with its rival, Fatah, the political organisation which controls the West Bank under Palestinian president Mahmoud Abbas' leadership.
Meanwhile, a 63 second-long siren rang midday in commemoration of the Nakba's 63rd anniversary.
Over 760,000 Palestinians - estimated today to number 4.7 million with their descendants - fled or were driven out of their homes in the conflict that followed Israel's creation.
Many took refuge in neighbouring Lebanon, Jordan, Egypt and elsewhere. Some continue to live in refugee camps.
About 160,000 Palestinians stayed behind in what is now Israeli territory and are known as Arab Israelis. They now total around 1.3 million, or some 20 percent of Israel's population.