TEL AVIV—Violence along the border between Syria and the Israeli-controlled Golan Heights escalated, claiming casualties on Sunday, as Israeli soldiers opened fire on demonstrators in two locations who sought to march across the frontier on the anniversary of the outbreak of the 1967 Arab-Israeli war.

Syria's state news agency said 23 people were killed and more than 350 were injured.

Israeli officials accused Syria of backing the protests to distract attention from its repression of weekslong antiregime protests, and said Lebanese authorities avoided a flare-up by declaring the border a closed military zone.

It was the worst day of fighting along the border since Israel and Syria fought a major war for control of the strategic Golan Plateau in 1973. Israel had reinforced the border with trenches, barbed wire and snipers amid reports of planned border protests. Israeli officials said they wanted to prevent a repeat of demonstrations two weeks ago, when hundreds of Palestinian demonstrators surprised and overwhelmed soldiers on the border and crossed into the Golan Heights.