JERUSALEM — Tens of thousands of Palestinians gathered along Gaza’s border with Israel on Friday to vent their pent-up frustration in a protest that quickly turned violent, with Israeli forces killing 15 at the border fence.

As many as 30,000 arrived early in the day at tent encampments on Gaza’s side of the fence to stage what was billed as the start of a peaceful, six-week sit-in. They were protesting against Israel’s longstanding blockade of the territory and in support of their claims to return to homes in what is now Israel.

But as some began hurling stones, tossing Molotov cocktails and rolling burning tires at the fence, the Israelis responded with tear gas and gunfire. The Israelis said they also exchanged fire with two gunmen across the fence and fired at two others who tried to infiltrate into Israel.