Thousands of Palestinians marched from Syria, Lebanon, the Gaza Strip and the West Bank toward Israeli border positions on Sunday, hurling rocks and surging across one frontier before the Israeli army opened fire, killing at least 12 people and injuring scores.

The marches, which brought the protest culture of the Arab Spring to Israel’s doorstep, marked an unprecedented escalation of the annual demonstrations on the anniversary of the establishment of Israel in 1948.

Prime Minister Binyamin Netanyahu said the demonstrators were challenging “the very existence of Israel” and, in nationally broadcast remarks, pledged that the country was “determined to defend our borders and sovereignty.”

Palestinians commemorate Israel’s founding as al-Naqba, or the catastrophe, marking the displacement of hundreds of thousands in the war that followed Israel’s declaration of independence.

The coordinated protests on Sunday were organized using many of the social media tools that have propelled revolts in Arab countries in recent months, and the message they carried, of Palestinian demands for the right to return to their ancestral homes, struck a raw nerve among Israelis, who have been watching the popular uprisings with concern that they could strengthen groups hostile to Israel.

Palestinians converged Sunday, the anniversary of Israel’s independence, in an annual protest.

Some Israeli officials pointed a finger at Syria and its ally Iran, accusing them of instigating the protests to deflect attention from the deadly repression of the anti-government demonstrations in Syria.

The Israeli army’s chief spokesman, Brig. Gen. Yoav Mordechai, said Syrian and Lebanese troops had failed to hold back the demonstrators, who had arrived in busloads from Palestinian refugee camps.

The most serious incident was on the border between Syria and the occupied Golan Heights, where thousands of protesters gathered on the Syrian side and hundreds flooded into the Israeli-held territory after flattening the border fence. Scores entered the Druze village of Majdal Shams, gathering in the central square, where they raised Palestinian flags.

“We cannot put up with this anymore. We are demanding our right of return,” said Muhammad Umran, 35, from the Yarmouk refugee camp in Syria’s capital, Damascus, who spoke by telephone from Majdal Shams. “We are not afraid,” he said, adding that his family was originally from a village near the city of Safed.

Another protester, Muhammad Suleiman, also from the Yarmouk camp, said by telephone that the crowds had passed through minefields and planted Syrian and Palestinian flags on an Israeli army jeep vacated by troops during the rock-throwing melee. The empty jeep was visible in television broadcasts from the scene.

Israeli troops opened fire to drive back the protesters, killing two, according to Israeli officials and Syrian television, which said that more than 100 were injured. The Syrian Foreign Ministry described the Israeli actions as “criminal acts.”

Soldiers later rebuilt the fence, and by nightfall nearly all the protesters were returned to Syria, an Israeli army spokeswoman said.

On the frontier with Lebanon, 10 people were killed and more than 100 were wounded, the Lebanese army said, when Israeli troops opened fire at a crowd of protesters that broke away from a larger demonstration and hurled rocks over the border fence near the village of Maroun al-Ras. The Israeli army said its forces fired warning shots when dozens of protesters tried to breach the fence and enter Israeli territory.

Hundreds of Lebanese troops, United Nations peacekeepers and riot police officers had been deployed in the border area, but there was also a presence of the guerrilla group Hezbollah, whose activists organized entry to Maroun al-Ras and handed out Palestinian flags, the Associated Press reported. Images from the scene broadcast on Hezbollah television showed mourners carrying the bodies of youths on stretchers through the crowds.

At the northern edge of the Gaza Strip, hundreds of protesters marched on a border crossing with Israel, and Israeli troops opened fire to drive them back.

Palestinians reported that the Israelis fired several artillery shells as a warning and then used gunfire to disperse the crowd. More than 100 people were wounded, according to local emergency services.

The army said soldiers had fired at the legs of rioters who hurled rocks at the troops and vandalized the border crossing.

In a separate incident along the Gaza border, Israeli troops fatally shot a Palestinian man who the army said was trying to plant explosives at the border fence.

At the Qalandia crossing between the West Bank and Jerusalem, stone-throwing youths battled soldiers for hours after a march by several hundred people was dispersed with tear gas. Palestinians reported that dozens were injured by rubber-coated bullets. A large rally was held in the central square in neighboring Ramallah, the seat of the Palestinian Authority. In Jordan, security forces used tear gas to disperse hundreds of demonstrators, preventing them from reaching the border with Israel, the Reuters news agency reported.

Inside Israel, where security forces were on high alert for possible violence, a truck driven by an Israeli Arab plowed through a crowded street in Tel Aviv, hitting several cars, crashing into a bus and killing a motorist. The police said that the driver claimed it was an accident, but that they were investigating whether the incident was a deliberate attack.

Special correspondent Samuel Sockol in the West Bank contributed to this report.