In scenes of mayhem, troops opened fire on peaceful pro-Morsi protesters. Islamists threw one opponent off a rooftop.
"God, make Morsi victorious and bring him back to the palace," Brotherhood chief Mohammed Badie proclaimed before cheering supporters at a Cairo mosque in his first appearance since the overthrow. "We are his soldiers; we defend him with our lives."
Badie said it was a matter of honor for the military to abide by its pledge of loyalty to the president, in what appeared to be an attempt to pull it away from its leadership.
"Your leader is Morsi. . . . Return to the people of Egypt," he said. "Your bullets are not to be fired on your sons and your own people."
Hours later, Badie's deputy, Khairat el-Shater, considered the most powerful figure in the organization, was arrested in a Cairo apartment along with his brother on allegations of inciting violence, said Interior Ministry spokesman Hani Abdel-Latif.
After the speech, a large crowd of Islamists surged across 6th October Bridge over the Nile toward Tahrir Square, where a giant crowd of Morsi's opponents had been massed all day. Battles broke out there and near the neighboring state TV building. Pro-Morsi youth shielded themselves from flying stones and fireworks with sheets of metal.
"They are firing at us, sons of dogs! Where is the army?" one Morsi opponent shouted as another was brought to medics with his jeans soaked in blood from leg wounds. At least three people were killed at the bridge.
The fighting ended when at least seven armored personnel carriers sped across the bridge, chasing away the Morsi supporters. Young civilians jumped onto the roofs of the APCs, shouting insults at the Islamists and chanting, "The people and army are one hand."
Across the country, clashes erupted as Morsi supporters tried to storm government buildings or military facilities, battling police or Morsi opponents. At least 30 people were killed throughout the day in Egypt, with 210 wounded, said Heath Ministry official Khaled el-Khatib.
Islamists descended on an anti-Morsi rally, opening fire in the coastal city of Alexandria, where at least 12 people were killed, mostly Morsi opponents, emergency services official Amr Salama said. One man was stabbed and thrown from the roof of a building by Morsi supporters after he raised an Egyptian flag and shouted insults against the ousted president.
The U.S. State Department condemned the violence and called on all Egyptian leaders to denounce the use of force and to prevent further bloodshed.