Israeli troops clashed with Arab protesters today along three hostile borders, including the frontier with Syria, leaving at least 12 people dead and dozens wounded.
The unprecedented wave of demonstrations marked a Palestinian day of mourning for their defeat at Israel's hands in 1948.
In the most serious incident, the Israeli military said thousands of protesters approached Syria's border with the Israeli-controlled Golan Heights and hundreds burst through the fence.
Soldiers opened fire to stop them, the military said. Dozens were wounded and four were reported to have been killed.
It was a rare incursion from the usually tightly controlled Syrian side, and Israeli officials accused Damascus of fomenting the violence in an attempt to divert attention from the deadly crackdown on protests within its borders against the rule of President Bashar Assad.
"The Syrian regime is intentionally attempting to divert international attention away from the brutal crackdown of their own citizens to incite against Israel," said Lt Col Avital Leibovich, an Israeli military spokeswoman.
Israeli media reported that the incident was over by early evening, but the military did not immediately confirm that.
Prime Minister Benjamin Netanyahu said he had ordered the military to act with "maximum restraint".
"But nobody should be mistaken. We are determined to defend our borders and sovereignty," he added in a brief address broadcast live on Israeli TV stations.
Today's unrest - which came after activists used Facebook and other websites to mobilise Palestinians and their supporters in neighbouring countries to march on the border with Israel - marked the first time the protest tactics that have swept the Arab world in recent months have been directed at Israel.
Deadly clashes also took place along Israel's nearby northern border with Lebanon, as well as in the Gaza Strip, near Israel's southern border. The Israeli military said 13 soldiers were lightly wounded in the Lebanon and Syria clashes.
In addition, hundreds of Palestinian threw stones at Israeli police and burned tyres at a checkpoint outside Jerusalem before they were dispersed.
The unrest came as the Palestinians marked the "nakba," or "catastrophe", the term they use to describe their defeat and displacement in the war at the time of Israel's founding in 1948.
Hundreds of thousands of Palestinians were uprooted, and the dispute over the fate of the refugees and their descendants, now numbering several million, remains a key issue in the Mideast conflict.
It also comes at a critical time for US Mideast policy. President Barack Obama's envoy to the region, George Mitchell, resigned on Friday, and the US president may now have to retool the administration's incremental approach to peacemaking. Obama is to deliver a Mideast policy speech this week.
Palestinian medics also said one person was killed and 65 others were wounded when demonstrators in the Gaza Strip tried to approach a heavily fortified border crossing into Israel.
A second Palestinian was killed in a separate incident. Israel's military said he was trying to plant a bomb along the fence.