At least 41 Palestinians have been killed and 1,800 wounded by Israeli troops in clashes on the Gaza border, Palestinian officials say.
The violence comes ahead of the opening of the US embassy in Jerusalem, which has infuriated Palestinians.
They see it as clear US backing for Israeli rule over the whole city, whose eastern part Palestinians lay claim to.
Top US officials, including President Donald Trump's daughter and her husband, are attending Monday's event.
The dedication ceremony is now under way.
What has happened at the border?
Palestinians hurled stones and firebombs while the Israeli military used snipers, as black smoke poured from burning tyres.
The Hamas-run health ministry said children were among those killed on Monday.
The mass demonstrations, led by Gaza's Islamist rulers, Hamas, are part of a six-week protest dubbed the "Great March of Return".
Israel's army said 35,000 Palestinians were taking part in "violent riots" along the security fence and that its troops were operating "in accordance with standard procedures".
The Israeli military said it had killed three people trying to plant explosives near the security fence in Rafah.
It said aircraft had also "targeted Hamas military posts near the Jabalia area after troops were fired upon".
What led to the Gaza protests?
Palestinians have held weekly protests in the run-up to their annual commemoration of what they call the Nakba or Catastrophe, when hundreds of thousands of their people fled their homes or were displaced following the foundation of the Israeli state on 14 May 1948.
Scores of Palestinians have been killed since the protests began. Thousands more have been wounded.
Hamas, which is in a state of conflict with Israel, had said it would step up protests in the lead-up to Tuesday, the official Nakba commemoration.
It says it wants to draw attention to what Palestinians insist is their right to return to ancestral homes in what became Israel.
"Today is the big day when we will cross the fence and tell Israel and the world we will not accept being occupied forever," a science teacher in Gaza, Ali, told Reuters news agency.