Israeli soldiers opened fired on pro-Palestinian protesters in Syria as they marched on the Golan Heights, which are occupied by Israel.
Syrian state television said that 18 people were killed and more than 200 wounded.
The incident occurred near the village of Majdal Shams at around noon on Sunday.
The protesters were marking the 44th anniversary of the 1967 Mid-East war, when Israel seized the Golan Heights from Syria in the closing stages of the war, along with the West Bank, Gaza and East Jerusalem.
Israel earlier vowed to prevent a repeat of a similar march last month, when hundreds of people breached the border fence.
Prime Minister Benjamin Netanyahu said he would not allow "extremists" to breach Israel's borders.
At least 12 people were killed by Israeli soldiers during demonstrations on 15 May.
They were marking the 63rd anniversary of Nakba, which means ''catastrophe'' in Arabic, when hundreds of thousands of Palestinians lost their homes amid the fighting after the creation of the state of Israel in 1948.