Thousands of protesters led by Palestinian militant leaders, including Hamas' crippled founder and spiritual mentor Sheik Ahmed Yassin, marched in Gaza city brandishing Palestinian flags and banners that denounced the barrier.
"Down with the wall. ... What a shame. ... Where are the freedom-loving peoples," read the banners.
Israel said it was building the barrier through Palestinian territory to deter suicide attacks, while Palestinians charged that the fence was aimed at annexing more Palestinian land.
Hamas official Mahmoud al-Zahar played down the importance of the expected World Court ruling over the legality of the barrier.
"It is just a media battle which we might win, thus boosting our stance in European and U.S. public opinion, and if we lose the case we won't be losing anything more than we have already lost," al-Zahar said.
Islamic Jihad official Mohammed al-Hindi said the only way to fight the barrier is through armed resistance against Israel and not through the hearing, which was requested by the United Nations.
Israel is staying away from the hearing. It has argued that the World Court lacks jurisdiction.
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