MAJDAL SHAMS, Golan Heights (AFP) – Hundreds of police fanned out across the Golan Heights on Monday in search of refugees who crossed over from Syria in some of the bloodiest violence in years along Israel's borders.

Twelve people were killed and around 300 others were injured on Sunday when Israeli troops fired on thousands of people along the Syrian and Lebanese borders, as well as in the Gaza Strip and the West Bank.

The protests came as Palestinians in the territories and in the neighbouring countries marked the anniversary of Israel's founding in 1948, in an event known in Arabic as the "nakba" or "catastrophe".

The Israeli army and police remained on high alert with hundreds of police working through the night in the Golan Heights town of Majdal Shams to find Syrian protesters.

"Throughout the night, police have been searching house-to-house for suspects who could still be in Majdal Shams," spokesman Micky Rosenfeld told AFP, saying police had detained a 34-year-old Syrian, who was trying to leave the town in a taxi driven by a Palestinian from east Jerusalem.

Brigadier General Yoav Mordechai told army radio that the military remained "in a state of high alert in the north, the south and the centre."

Defence chiefs also extended a 24-hour lockdown on the occupied Palestinian territories which had been due to end at midnight on Sunday.

The breach in the Israeli-occupied Golan Heights was one of the worst incidents of violence there since a 1974 truce accord, while the clashes along the Lebanese border marked the bloodiest confrontation since the 2006 war between the two neighbours.

Most of the victims were in Lebanon, where 10 people were killed and 110 injured when Israeli troops opened fire on people trying to scale the border fence.

Hundreds of protesters from Syria also entered the Israeli-occupied Golan Heights, prompting a volley of fire that left at least two people dead, medics said. The army said "dozens" had been injured.

Thirteen soldiers were also injured in the two incidents, the military said.

In Gaza, 125 people were injured, five of them seriously, when troops opened fired as more than 1,000 Palestinians marched on the northern Erez crossing.

Elsewhere, 29 others were injured in clashes in the West Bank and annexed east Jerusalem.

Palestinian camps across Lebanon declared a day of mourning, with shops observing a general strike ahead of the funerals for the 10 victims, which were taking place in four refugee camps on Monday.

Hassan Nasrallah, head of the Lebanese Shiite militia group Hezbollah praised the protesters, saying they had "given the nakba new meaning," and Hamas chief Khaled Meshaal, speaking from Damascus, condemned Israel's "new Zionist crime against our people who were demonstrating peacefully."

Lebanon has filed a complaint with the United Nations, urging it to make Israel "halt its aggression and provocation" while Syria warned that the Jewish state would bear full responsibility for its "criminal" actions.

Israel responded by saying it would file its own complaint against both Syria and Lebanon, accusing them of violating Israeli borders.

UN Secretary General Ban Ki-moon expressed deep concern over the violence and urged all sides to show the "utmost responsibility" to avoid new hostilities, a spokesman said.

Filippo Grandi, head of the UN agency for Palestinian refugees (UNRWA) also condemned the bloodshed, saying it proved the need for a solution to the plight of the refugees, who now number 4.8 million.

"I deplore the deaths of Palestine refugees in Lebanon, the occupied Golan Heights and the occupied Palestinian territory," he said in a statement.

But Israeli Prime Minister Benjamin Netanyahu said the Jewish state was "determined" to defend its borders against protestors bent on denying Israel's right to exist.

On Monday morning, an Israeli court extended the detention of a 22-year-old Arab Israeli truck driver accused of ploughing into 12 cars and a bus on Sunday, killing one, in what police believe was a deliberate attack.

More than 760,000 Palestinians -- estimated today to number 4.8 million with their descendants -- were pushed into exile or driven out of their homes in the conflict that accompanied the Jewish state's foundation.