At least 62 Palestinians were killed and more than 2,700 wounded by Israeli troops in clashes on the Gaza border, BBC reported, citing Palestinian officials.
The violence erupted as the US Embassy was opening in Jerusalem, which has infuriated the Palestinians.
They see it as clear US backing for Israeli rule over the whole city, whose eastern part Palestinians lay claim to.
Turkey’s President Recep Tayyip Erdoğan accused Israeli authorities of carrying out a genocide against Palestinians, as Turkish government declared a three day national mourning over the incident.
Top US officials, including President Donald Trump’s daughter and her husband, are attending Monday’s event.
Gaza’s Islamist rulers, Hamas, have led mass protests in a “Great March of Return” for the past six weeks. Israel says demonstrators are trying to breach the border fence.
Children were among those killed on Monday, the Hamas-run health ministry says.
Palestinians hurled stones and firebombs, while the Israeli military used snipers as black smoke poured from burning tires.
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.”