CAIRO—Egypt's government declared the Muslim Brotherhood a terrorist organization on Wednesday, outlawing all its activities, financing and membership a day after accusing the Islamist group of a bombing that killed 16 people.

The Brotherhood firmly denied involvement in the apparent suicide car bombing in the early hours of Tuesday. The blast targeted a police headquarters in the Nile Delta city of Mansoura.

The once-powerful Brotherhood, which dominated all elections after the 2011 uprising that ousted Egypt's longtime authoritarian regime, had already been banned as a political party and a nongovernmental organization.

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