JERUSALEM — 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 Barack Obama's envoy to the region, George Mitchell, resigned Friday after more than two years of fruitless efforts. The U.S. president may now have to retool the administration's approach to peacemaking. 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.
Top stories: Turmoil in the Middle East
Sunday's unrest — which came after activists used Facebook and other websites to mobilize Palestinians and their supporters in neighboring countries to march on the border with Israel — 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 from Syria in the 1967 war, and Syria demands the area back as part of any peace deal. Israel has annexed the territory. 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 forces did not intervene — and 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.
The Israeli army said more than 100 people were sent back to Syria by the time the unrest died down several hours later.
Israeli defense officials, speaking on condition of anonymity because of the sensitive nature of the information, acknowledged the military was caught off guard by the violent marches.
Officials also said there were strong signs that Syria and its Iranian-backed Lebanese ally, Hezbollah, orchestrated the unrest.
"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.
Israel's military spokesman, Brig. Gen. Yoav Mordechai, told Channel 2 TV he also saw "fingerprints of Iranian provocation and an attempt to use 'nakba day' to create conflict."
Hezbollah's al-Manar TV was in place to film much of the day's clashes, and defense officials said the activists were bused in from Palestinian refugee camps throughout Syria. Many of them held European passports and told interrogators they had been flown in from abroad for the march.
"It's our land," one of the infiltrators, Sufian Abdel Hamid, told Israel's Channel 2 TV. "We won't stop trying to come back."
An explosion of unrest along the border could play into the hands of Syrian President Bashar Assad, who has faced two months of popular protests against political repression and rights abuses in his country. The uprising, in which human rights groups say more than 800 people have been killed, is the most serious challenge to the Assad family's 40-year dynasty.
Assad has cast himself as the only person who can bring stability to Syria — a country with a volatile mixture of religions and sects, and with a hostile neighbor in Israel.
About 25 miles (40 kilometers) to the west, Israeli troops clashed with a large crowd of Lebanese demonstrators who approached that border. The military said it opened fire when protesters tried to damage the border fence. Security officials in Lebanon reported 10 dead.
It was the deadliest incident along the volatile border since Israel fought Lebanese Hezbollah guerrillas during a monthlong war five years ago.
Sunday's shooting erupted at the tense border village of Maroun el-Rass, which saw some of the fiercest fighting in 2006. Thousands of Palestinian refugees traveled to the village in buses adorned with posters that said: "We are returning." Many came from the 12 crowded refugee camps in Lebanon where some 400,000 Palestinian refugees live.
Hundreds of Lebanese soldiers, U.N. peacekeepers and riot police deployed heavily in the area, taking up positions along the electrified border fence and patrolling the area in military vehicles. Young Hezbollah supporters wearing yellow hats and carrying walkie-talkies organized the entry to the village and handed out Palestinian flags.
There was also violence in a predictable location — Gaza.
Palestinian medics said 125 people were wounded when demonstrators in the Gaza Strip tried to approach a heavily fortified border crossing into Israel. One man was killed by an Israeli sniper. The military said he was trying to plant a bomb.
In Jordan, meanwhile, police blocked a group of protesters trying to reach the border with Israel. In addition, hundreds of West Bank Palestinian threw stones at Israeli police and burned tires at a checkpoint outside Jerusalem before they were dispersed.
Inside Israel, police were on high alert for disturbances among the country's large Arab minority, and Israeli police spokeswoman Sigal Toledo said a deadly traffic incident involving an Arab truck driver in Tel Aviv was "most likely" an attack.
The truck plowed through a crowded street, crashing into a bus, several cars and pedestrians, killing one and injuring 16 others. Police said the 22-year-old driver claimed it was an accident, but a witness said he had to subdue the man and that he was shouting slogans against Jews.
The Associated Press and Reuters contributed to this report.