CAIRO - At least 72 people were killed during clashes in Cairo on Saturday, Egypt's health ministry said, after violence erupted at a demonstration in support of ousted president Mohamed Mursi.
The ministry said nine others had died in violence in Egypt's second city, Alexandria, a day earlier, putting the toll in two days of unrest at 81.
The Cairo bloodshed was the worst since Mursi's ouster in a military-led coup on July 3 and prompted domestic and international condemnation, as protesters accused security forces of using live ammunition.
US Secretary of State John Kerry, whose country contributes millions of dollars in military and economic assistance to Egypt, expressed Washington's "deep concern" about the bloodshed.
In a statement, Kerry also said the authorities had "a moral and legal obligation to respect the right of peaceful assembly and freedom of expression".
Ahmed Aref, a spokesman for Mursi's Muslim Brotherhood, said 66 people were killed at Cairo's Rabaa al-Adawiya mosque, where supporters of the ousted president have been camped demanding his reinstatement.
An AFP correspondent counted 37 bodies in an Islamist-run field hospital at the mosque, and the emergency services said other hospitals received an additional 29 corpses.
Mursi's supporters said security forces had opened fire on unarmed protesters, but Egypt's interior ministry insisted that only tear gas was used and blamed the clashes on Islamists.
Interior Minister Mohammed Ibrahim had warned that pro-Mursi demonstrations would be dispersed "in a legal fashion" and "as soon as possible". He called on them to "come to their senses" and go home.
A Muslim Brotherhood statement however, condemning what it called an "inhuman massacre", said this had only strengthened their determination to peacefully resist the "coup d'etat".
Saturday's violence came after mass rival demonstrations in Cairo and other parts of Egypt.
State media and the interior ministry said police had fired tear gas at stone-throwing protesters on the airport road, but witnesses told AFP that security forces fired live bullets.
By midday, medical workers began ferrying bodies wrapped in white shrouds to hospitals, carrying them on blood-soaked stretchers past furious Mursi loyalists.
Some wept and women ululated as each body was taken from the makeshift morgue in a marble-floored section of the mosque.
The health ministry said 748 people had been injured on Friday and Saturday, including 269 at Rabaa al-Adawiya.
The Cairo violence was the deadliest since 53 Mursi supporters were killed outside the Republican Guard headquarters in the capital on July 8.
Sheikh Ahmed al-Tayyeb, the grand imam at Al-Azhar, Egypt's top Sunni Muslim authority, condemned the violence, calling for an "urgent judicial investigation" and for those responsible to be punished "regardless of their affiliation."
Vice President Mohamed ElBaradei, who joined the transitional government that replaced Mursi's administration, denounced what he called "excessive use of force."
The National Salvation Front, a coalition of leftist and liberal groups, expressed "grief" over the deaths, but said Mursi's Brotherhood bore blame for its "provocative approach."
British Foreign Secretary William Hague urged "Egyptian authorities to respect the right of peaceful protest, to cease the use of violence against protesters, including live fire, and to hold to account those responsible".
The European Union called for "a rapid move to an inclusive transformation process" that would include the Brotherhood.
But Egypt's interior ministry said police "did not use more than tear gas" and accused Islamists of firing on the security forces, wounding 14 policemen, two in the head.
By Saturday evening, the mood in Rabaa al-Adawiya was largely calm. Demonstrators had draped new banners with the word "peaceful" written in Arabic and English around the area.
The bloodshed came after army chief General Abdel Fattah al-Sisi, who led the coup that ousted Mursi, called for a mass show of support for a crackdown on "terrorism."
Hundreds of thousands of anti-Mursi protesters obliged, thronging Cairo's Tahrir Square and around the Ittihadiya presidential palace on Friday.
But Mursi supporters said their turnout showed many "reject the bloody, military fascist coup that wants to set the wheel of history back".
On Friday, authorities remanded Mursi in custody for 15 days, accusing him of the "premeditated murder of some prisoners, officers and soldiers" when he broke out of prison during the 2011 uprising that toppled veteran strongman Hosni Mubarak, state news agency MENA said.
He also stands accused of conspiring to "storm prisons and destroy them... allowing prisoners to escape, including himself".
The military has so far kept Mursi's whereabouts secret to avoid attracting protests by his supporters.
Egypt, the Arab world's most populous country, has been rocked by violence that has now killed more than 300 people in the less than four weeks since the coup.
Political polarisation has raised fears of prolonged violence, and even a militant backlash, including in the Sinai Peninsula, where the army is already facing daily attacks.