Palestinians in Gaza have marched in their thousands to the Israeli border at the start of a six-week protest.
The Palestinian health ministry says at least 15 people have been killed and 750 wounded, many of them by Israeli gunfire.
The Israeli military reported "rioting" at six places and said it was "firing towards main instigators".
Palestinians have pitched five camps near the border for the protest, dubbed 'the Great March of Return'.
They are demanding that refugees be allowed to return to homes that are now in Israel.
The Israel Defense Forces (IDF) said there were about 17,000 Palestinians in five locations along the border fence. It said it had "enforced a closed military zone" in the area surrounding Gaza.
The IDF said troops were "firing towards the main instigators" to break up rioting that included petrol bombs and stones being thrown at the fence.
A spokesman said all those who were killed had been trying to breach or damage the border fence, the Jerusalem Post reported.
"Live ammunition was used only against those attempting to harm the fence," Brig Gen Ronen Manelis said.
Israel has deployed tanks and snipers. Witnesses said a drone had been used in at least one location to drop tear gas.
Hamas, the militant group that has controlled the Gaza Strip since 2007, does not recognise Israel's right to exist, but last year said it was ready to accept an interim Palestinian state limited to Gaza and the West Bank.
Addressing protesters on Friday, Hamas senior political leader Ismail Haniyeh said "we will not concede a single inch of the land of Palestine".
He said: "There is no alternative to Palestine and no solution except to return."
The Jerusalem Post also reported a rare public appearance by the leader of Hamas in Gaza, Yehiya Sinwar.
The Israeli military oversees a no-go zone along the Gaza border, citing security concerns, and has doubled its troop presence for the protest. It fears the protest could be an attempt at a mass breach of the border.
Israeli commander Maj Gen Eyal Zamir said: "We are identifying attempts to carry out terror attacks under the camouflage of riots. We urge citizens to stay away."
The IDF also accused Hamas of endangering a seven-year-old girl by sending her across the border.
It said that Israeli troops, "realising she was a child", safely returned the girl to her family. It accused Hamas of "cynically" using women and children, "sending them to the security fence and endangering their lives".
The Israeli foreign ministry has said the protest is a "deliberate attempt to provoke a confrontation with Israel" and that "responsibility for any clashes lies solely with Hamas and other participating Palestinian organisations".
Defence Minister Avigdor Lieberman took the unusual step of tweeting in Arabic, accusing Hamas of playing with citizens' lives and warning people not to take part.
What is the protest about?
Palestinians have erected five main camp areas along the Israel border for the protest, from Beit Hanoun in the north to Rafah near the Egyptian border.
The Great March of Return protests were starting on Friday, as 30 March marks Land Day, which commemorates the killing of six protesters by Israeli security forces during demonstrations over land confiscation in 1976.
The protest is scheduled to end on 15 May, which Palestinians call Nakba (catastrophe) and which marks the displacement of hundreds of thousands of Palestinians during the conflict surrounding the creation of Israel in 1948.
Palestinians have long demanded their right to return but Israel says they should settle in a future Palestinian state in Gaza and the West Bank.