Brotherhood leaders vow to appeal the court ruling, and supporters say they will continue their demonstrations, which have persisted despite a state of emergency throughout much of the country.

Rights activists also expressed dismay over what they fear may become an even greater polarization of the country.

“We've moved into a phase where there seems to be an overall political decision to exclude the brotherhood from Egypt's political future," said Heba Morayef of Human Rights Watch. "And I think without some form of inclusion of the brotherhood, I don't see political stability occurring in Egypt's future.”

A spokeswoman for the U.S. State Department said Secretary of State John Kerry has discussed the matter with Egyptian Foreign Minister Nabil Fahmy.

"A transparent and inclusive political process that preserves the rights of all Egyptians to participate and leads back to a civilian-led government is critical to the success of Egypt's political and economic future," said Jen Psaki.