MAJDAL SHAMS, Golan Heights – Mobilized by calls on Facebook, thousands of Arab protesters marched on Israel’s borders with Syria, Lebanon and Gaza on Sunday in an unprecedented wave of demonstrations, sparking clashes that left at least 15 people dead in an annual Palestinian mourning ritual marking the anniversary of Israel’s birth.

In a surprising turn of events, hundreds of Palestinians and supporters poured across the Syrian frontier and staged riots, drawing Israeli accusations that Damascus, and its ally Iran, orchestrated the unrest to shift attention from an uprising back home. It was a rare incursion from the usually tightly controlled Syrian side and could upset the delicate balance between the two longtime foes.

Prime Minister Benjamin Netanyahu, who heads to Washington at the end of the week, said he ordered the military to act with “maximum restraint” but vowed a tough response to further provocations.

“Nobody should be mistaken. We are determined to defend our borders and sovereignty,” he declared in a brief address broadcast live on Israeli TV stations.

The violence showed Israel the extent of Arab anger over the Palestinian issue, beyond the residents of the West Bank and Gaza, and came at a critical time for U.S. Mideast policy.

President Obama’s envoy to the region, former Maine Sen. George Mitchell, resigned Friday after more than two years of fruitless efforts. Obama is expected to deliver a Mideast policy speech in the coming week.

Deadly clashes also took place along Israel’s nearby northern border with Lebanon, as well as in the Gaza Strip on Israel’s southern flank. The Israeli military said 13 soldiers were wounded, none seriously.

Sunday’s unrest came after activists used Facebook and other websites to mobilize Palestinians and their supporters in neighboring countries to march on the border with Israel. It marked the first time the protests that have swept the Arab world in recent months have been directed at Israel.

The events carried a message for Israel: Even as it wrestles with the Palestinian demand for a state in the West Bank, Gaza and east Jerusalem — areas Israel captured in the 1967 Mideast war — there is a related problem of neighboring countries that host millions of Palestinians with aspirations to return.

The fate of Palestinian refugees is one of the thorniest issues that any Israeli-Palestinian peace deal will have to address.

Palestinians were marking the “nakba,” or “catastrophe” — the term they use to describe their defeat and displacement in the war that followed Israel’s founding on May 15, 1948. Hundreds of thousands of Palestinians were uprooted. Today, the surviving refugees and their descendants number several million people.

Each year, Palestinians throughout the region mark the “nakba” with demonstrations. But never before have marchers descended upon Israel’s borders from all directions. The Syrian incursion was especially surprising.

Israel captured the Golan Heights from Syria in the 1967 war, and Syria demands the area back as part of any peace deal. Despite hostility between the two countries, Syria has carefully kept the border quiet since the 1973 Mideast war.

Around midday, thousands of people approached the frontier, hoisting Palestinian flags, shouting slogans and throwing rocks and bottles at Israeli forces. When hundreds of people burst across the border fence into the Israeli-controlled town of Majdal Shams, surprised soldiers opened fire. Syrian officials reported four people were killed, and dozens wounded.

Rioters paraded through the town, flashing Syrian ID cards and holding Palestinian flags.

“This was a surprise for everyone. I have been here my whole life and never saw anything like this,” said Khatib Ibrahim, a 51-year-old resident who watched the clashes unfold as he worked in his family’s grove.