Zamalek failed to maintain their unbeaten run in the African Confederation Cup after slumping to a 1-0 away defeat by DR Congo's Sanga Balende in a controversial match on Sunday.
The Cairo giants were stunned by an early goal from Musema Ngowa, who scored what proved to be the winner after 13 minutes.
Zamalek piled on pressure after falling behind to level the score but they had three goals from Ahmed Tawfik, Ahmed Ali and Bassem Morsi disallowed by the referee, prompting protests from the Zamalek management.
It was Zamalek's first defeat under Portuguese coach Jesualdo Ferreira, who has guided Zamalek in ten matches in all competitions so far.
"I have never seen such bad refereeing in my life," Zamalek's board member Rehab Abou Regeila told Nile Sports after the game, which was not televised.
"The referee disallowed a legitimate goal from striker Ahmed Ali. The assistant referee ran to the center of the pitch in a sign that the goal is legitimate, but he himself was stunned by the referee's decision.
"I cannot also understand why the referee had not allowed Bassem Morsi's goal to stand," he added.
The Zamalek official revealed that the club will file a complaint against the referee after "his bad performance handed the team an undeserved loss".
Zamalek will have to overturn the first-leg deficit in Suez in a fortnight to reach the group stage of Africa's secondary club competition.
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