At least five Palestinians killed and 25 others wounded in an explosion next to the separation fence along the Israeli frontier.

The cause of the blast was not immediately known.

Wednesday’s 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 organised by Hamas, the Palestinian group that has governed the territory since 2007.

The demonstrators said Israeli soldiers fired tear gas before the deadly blast.

Witnesses told local media that when a Palestinian Explosives Engineering Unit was trying to defuse an explosive device, Israeli forces opened fire, preventing them from escaping the blast.

The Israeli army, which has carried out several spells of deadly bombing on the besieged enclave, 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 tyres 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 enclave of two million people has remained under Israeli land, sea and air blockade with tight control over the movement of people and goods.

Israel has justified the harsh measures saying it is needed to keep Hamas from arming.