Palestinian Authority President Mahmoud Abbas declared Saturday a day of mourning after at least a dozen Palestinian protesters died at the Gaza-Israel border on Friday.

Tens of thousands of Palestinians gathered at the 40-mile border on Friday in preparation for a six-week protest demanding the right of return for refugees and descendants of present-day Israel.

On Friday, over a thousand Palestinians were injured at the border on “Land Day,” which commemorates the 1976 deaths of six Arab protesters who were challenging land confiscations in Israel, The Jerusalem Post reported.

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The Israeli military claims the protesters were rioting in six different areas along the border and the government has dismissed the demonstrations as a ploy for military action by the terrorist group Hamas, according to The Guardian.

Thirteen Palestinians were wounded at the Gaza-Israel border on Saturday after Israeli troops fired warning shots towards Palestinians, according to Reuters.

Abbas has also called for a general strike in the West Bank region.

The State Department said on Friday that the U.S. is “deeply saddened” by the deaths of Palestinian protesters in Gaza.