Nabil Amr, a long-time political ally of Mahmoud Abbas, has described the Palestinian Authority president as a "third-world tyrant."
Amr also joined a long list of top Fatah officials who have accused Abbas of stealing the vote during the recent elections for the faction's Central Committee and Revolutionary Council.
Amr's attack on Abbas came as a surprise to many Palestinians and is a sign of the continued squabbling within Fatah.
Until last week, Amr was one of Abbas's closest advisors. About two years ago, Abbas appointed him as PA ambassador to Egypt. Before that, he served as minister of information.
Last year, Amr was entrusted with establishing Fatah's first satellite TV station - a multi-million dollar project that is still far from materializing due to lack of funds.
Last month, Amr was appointed as spokesman for Fatah's Sixth General Assembly that took place in Bethlehem.
Amr was among dozens of candidates who sought to be elected as members of Fatah's key decision-making body, the Central Committee. But when the results were announced, Amr was surprised to learn that, contrary to promises he had received from Abbas, he hadn't been elected.
In his capacity as spokesman of the conference, Amr repeatedly denied allegations made by many delegates that Abbas and his senior aides had stolen the vote. Then, Amr insisted that the vote was fair and honest.
However, upon learning that he hadn't fulfilled his promise that he be elected as member of the Central Committee, Amr announced his resignation from all PA and Fatah institutions.
At a meeting of PLO leaders chaired by Abbas in Ramallah last week, Amr lashed a scathing attack on the PA president, dubbing him a "third-world tyrant" and accusing him of forging the results of the Fatah elections.
A PLO official who attended quoted Amr as accusing Abbas of trying to seize control over the PLO "the same way he did with Fatah."
The official said that Amr later apologized for personally insulting Abbas and announced that he was quitting all his jobs and leaving the country.
Amr said on Wednesday that his decision to quit was the result of "basic differences" with Abbas over a variety of issues. He said that he resigned as spokesman for the Fatah congress in Bethlehem even before the vote was held for the Central Committee.
Amr claimed that Abbas had intervened to make sure that he wasn't elected as member of the Central Committee. "When I realized that Abbas was deliberately foiling my efforts to be re-elected, I walked out of the conference and resigned as spokesman," Amr told the London-based Al-Quds Al-Arabi.
Amr added that Abbas made sure that all his preferred candidates were elected "and that's why I decided to leave in a way that preserved my dignity."
Amr is the second senior Fatah official to talk about fraud during the elections for the two Fatah bodies. The other Fatah official is Ahmed Qurei (Abu Ala), who declared immediately after the results were announced that the fraud in the recent Iranian presidential elections were nothing compared to what happened in Bethlehem.
Like Amr, Qurei also pointed an accusing finger at Abbas because he was not elected to the Central Committee.
But Qurei has since refrained from repeating the allegations, especially after Abbas compensated him by making sure he was elected as member of the PLO's Executive Committee.
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