CAIRO — Three activists who played central roles in the uprising against former President Hosni Mubarak were convicted on Sunday of participating in recent protests and sentenced to three years in prison, raising fears that the new government was seeking revenge against opponents of Egypt’s old order.

Human rights advocates said the harsh sentences were the first verdicts in a political case against non-Islamists since the military ousted President Mohamed Morsi of the Muslim Brotherhood in July. The court’s decision was the clearest strike yet against leading figures of the January 2011 uprising by the new military-backed government, which has empowered the security agencies that were among the main targets of the protesters.

The sentences follow new charges against Mr. Morsi and his allies for their roles in stirring unrest during the revolt.

Gamal Eid, the executive director of the Arabic Network for Human Rights Information, said there was little doubt that Egypt’s military-backed leaders intended the latest sentences as a warning to pro-democracy activists. “It is time to shut up, to stay quiet,” Mr. Eid said, summing up the message. “There is only one choice — to support the military or to be in jail.”