GAZA CITY, Gaza Strip (AP) — At least four Palestinians were killed and 20 others wounded Wednesday in an explosion next to the separation fence along the Israeli frontier with Gaza, Palestinian health officials said.

The cause of the blast wasn’t immediately known.

The explosion took place during a demonstration along the fence marking the anniversary of Israel’s withdrawal from the Gaza Strip in 2005. The event on Gaza’s eastern border was organized by Hamas, the Islamic militant group that has governed the territory since 2007.

The Israeli army, which has fought four wars against Hamas, denied involvement. It said that demonstrators were trying to throw a bomb over the fence when the device detonated. It released an aerial video showing a blast along the fence. Debris flew into the air, and several people could be seen running away.

Earlier in the day, participants held Palestinian flags and burned tires to celebrate the anniversary of Israel’s withdrawal. Suhail al-Hindi, a Hamas leader, praised the end of what he described as “this cruel Israeli occupation from Gaza.”

The demonstration turned violent. The army said that demonstrators threw grenades and other explosives across the border, while soldiers responded with tear gas before the deadly blast.

Hamas seized control of Gaza from the internationally recognized Palestinian Authority in 2007, a year after winning parliamentary elections.

Israel and Egypt have maintained a stifling blockade on the territory since the Hamas takeover, in a measure that Israel says is needed to keep Hamas from arming. The blockade, which restricts movement of goods and people in and out of Gaza, has ravaged the local economy.