The Muslim Brotherhood has vowed to remain on the streets in "peaceful protest" after a night of violence and mass rallies in which at least 23 people were killed and hundreds more injured.

A doctor at a field hospital said at least 16 people were killed in the early hours of Saturday, in a gun attack on supporters of the deposed president Mohamed Morsi in Nasr City, Cairo. Many more were wounded, suffering gunshot wounds and from the effects of tear gas.

Doctors said the death toll could rise sharply and a Muslim Brotherhood spokesman said 70 people had been killed.

In Alexandria, seven people were earlier reported killed and hundreds injured in clashes between supporters and opponents of Morsi.

The violence came on a night of protests called for on Wednesday by the army chief, General Abdel Fattah el-Sisi, to create a mandate against what he called "terrorism" by opponents of the coup against Morsi.

An Al Jazeera correspondent reporting from the field hospital in Nasr described a scene of chaos.

"I am stood in a room 20ft square, there are 20 to 30 beds lined up and every two to three minutes someone is brought through the door covered in blood," he said. "There is evidence of live fire, puncture wounds. There is blood all over the place. It's chaotic. These are rudimentary facilities ... no means to deal with bullet wounds."

Ahmed Aref, a spokesman for the Muslim Brotherhood, which backs Morsi, told Al Jazeera that it would continue peaceful demonstrations across Egypt, and refused to engage in negotiations with the interim government unless it stepped down from power.

The interim government hours earlier had warned Brotherhood supporters and those opposed to the military takeover that they would be removed from the streets "in a legal manner".

Police fired tear gas at protesters who had attempted to block the October 6 Bridge in Nasr City, and also moved in to disperse an anti-coup sit-in at Qaed Ibrahim mosque in the city. About 200 people remained inside on Saturday morning.

In a television interview the interim president, Adly Mansour, said that the government "cannot accept security disorder, cutting roads and bridges. The state has to impose order by all force and decisiveness." The interior minister, Mohamed Ibrahim, echoed Mansour's words, stating that protests against military rule would be dispersed "in a legal manner".

Pro-military rallies were larger and free of violence, with a mood of jubilation among the crowds. The main pro-army protests took place in Cairo's iconic Tahrir Square and outside the presidential palace.

"The people, the source of all power, mandate the army and police to purge terrorism," read a banner stretched across the entrance to Tahrir.

Tanks were guarding the entrance to the square, military helicopters buzzed overhead, and police officers mingled with the crowd, posing for photographs. It was all a far cry from the usual scene over the past two-and-a-half years, when protesters in Tahrir routinely clashed with police and denounced the army.

"The army are here to protect the people, they don't lie," said Ezzat Fahmi, 38, who said Sisi called the rallies "to show the entire world that the Egyptian people don't want the Brotherhood anymore."

Leaders of Tamarod, the petition campaign that collected signatures calling for Morsi to be toppled , endorsed Friday's protests, encouraging their supporters to help "cleanse Egypt". But some revolutionary movements, including the April 6 youth movement, rejected Sisi's call for rallies.

The pro-army rallies were intended to overwhelm the pro-Morsi crowd's numbers and demonstrate public support for the army. It has also raised fears - and hopes, among some Egyptians - that the army is planning a major crackdown on the Brotherhood.

"It is either victory over the coup or martyrdom," said Mohamed el-Beltagy, a leading member of the Muslim Brotherhood, in a speech to the crowds.

Earlier on Friday, the state news agency reported that Morsi is being held for 15 days while prosecutors investigate claims that he conspired with the Palestinian group Hamas to carry out attacks during Egypt's 2011 revolution.

A spokesman for the Brotherhood said the charges showed the "complete bankruptcy of the leaders of the bloody coup".