CAIRO — Violent clashes spread across Egypt on Wednesday after security forces stormed two sprawling protest camps in an early morning assault that killed scores of supporters of ousted president Mohamed Morsi.

With at least 281 people killed, it was the deadliest day in Egypt since the 2011 uprising that toppled autocrat Hosni Mubarak, and the fallout dealt a further blow to the prospect that the country might resume its path toward democracy. At least 37 died in clashes in the conservative oasis town of Fayoum.

By nightfall, the military-backed interim government that replaced Morsi after a July 3 coup, had declared a state of emergency, and Mohamed ElBaradei, the Nobel Peace Prize winner and vice president, had tendered his resignation in protest over the bloody crackdown.

The United States strongly condemned the violence and said it would hold the interim government accountable for its promises of a speedy transition to a democratically elected civilian administration.

Hundreds of Morsi-allied Muslim Brotherhood members were arrested nationwide after the dawn assault, the Egyptian government said. The authorities blamed the Islamist group for the violence and said police had confiscated guns, ammunition and other weapons from the protest sites.

The morning assault brought bulldozers crashing through protesters’ tents as security forces opened fire through clouds of smoke and tear gas. Witnesses later posted footage showing dozens of bodies lining the rooms of a makeshift hospital run by Morsi supporters outside the Rabaa al-Adawiya mosque.

Mohamed el-Beltagi, a top Brotherhood politician whose teenage daughter was among those killed, said security forces had sacrificed their legitimacy by carrying out the attack, and he demanded that any soldier “must take off his uniform’’ or be considered a “tool” of the government. He warned that the spreading violence could quickly turn Egypt into a new Syria, where an ongoing conflict has killed more than 100,000 people.

Egypt’s interim interior minister said Brotherhood supporters later stormed several provincial headquarters across the country and set at as many as seven Coptic Christian churches ablaze. The sectarian attacks reflect Islamist anger over the strong backing to the military shown by many members of Egypt’s Christian minority.

The sprawling tent cities of men, women and children had been erected by Brotherhood supporters to demand the reinstatement of Morsi, Egypt’s first democratically elected president. On Wednesday evening, the interim prime minister and the interior minister said security forces had acted with the utmost “self-restraint” after six weeks of unauthorized sit-ins by Morsi’s supporters.

The government said 43 police officers had been killed in the clashes. In addition to scores of protesters, the dead included at least two journalists, including a British cameraman for Britain’s Sky News network.

The large-scale arrests of Muslim Brotherhood members echoed the authoritarian approach adopted by Egypt during the Mubarak era, when the Islamist group was banned and heavily repressed. The Brotherhood’s Freedom and Justice Party, which backs Morsi, had emerged after Mubarak’s fall as the country’s strongest political force. But the group’s popularity plummeted under Morsi, as Egyptians complained of a sinking economy and little political reform.