The European Union on Monday condemned the killing of 42 supporters of Egypt's ousted president and said it was keeping its billions of euros in aid pledged to the country "under constant review".

"We are doing all that we can through talking to everyone on the ground to make sure everyone understands the need for peace to be maintained," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, said at a routine press briefing.

Iran deems "unacceptable" the Egyptian army's toppling of the country's first freely elected president Mohammed Morsi, the foreign ministry said on Monday.

"The intervention of armed forces in political affairs is unacceptable and disturbing," ministry spokesman Abbas Araqchi told the Mehr news agency, when asked about the developments in Egypt.

Gaza's Islamist Hamas rulers on Monday condemned the "massacre" of dozens of people demonstrating against last week's military coup which ousted former Egyptian president Mohammed Morsi, calling for an end to the bloodshed.

"Hamas condemns the massacre of dozens of peaceful Egypt civilians today at dawn, expresses its profound pain and sadness over the victims, and calls for an end to the bloodshed among the Egyptian people," it said in a statement.

Turkey's foreign minister on Monday denounced the killing of 35 activists in Egypt who demonstrated against last week's military coup that unseated president Mohammed Morsi.

"I strongly condemn the massacre during the morning prayers on behalf of the fundamental values of humanity that we have defended," Ahmet Davutoglu wrote in a message on Twitter.

Egypt's Salafist al-Nur party said on Monday that it had withdrawn from talks on the formation of a new government in response to the killing of protesters calling for ousted president Mohammed Morsi to be reinstated.

"We have decided to withdraw immediately from all negotiations in response to the massacre outside the Republican Guard" headquarters, al-Nur spokesman Nadder Bakkar said on Twitter.

Egypt's interim leader Adly Mansour launched an investigation into violence in Cairo that killed 42 people on Monday during an Islamist demonstration calling for the army to restore Mohammed Morsi as president.

"The president of the republic forms a judicial commission to investigate the events at the Republican Guard" headquarters, state television reported.

Gunmen shot dead an Egyptian soldier on Sunday during an attack on a checkpoint in the restive north of Sinai, a police official said.

The attack took place near the town of El-Arish, where Islamists this week stormed the provincial headquarters and raised the banner of Islamist militants.

Opponents of Egypt's deposed Islamist president Mohamed Morsi packed Tahrir Square in their tens of thousands on Sunday to show the world his ouster was not a military coup but the reflection of the people's will.

Staged as a counter-demonstration two days after Islamist rallies exploded into deadly violence, the protest raised the stakes as the country's interim leaders struggled to put together a new government.

Egypt's army had no choice but to overthrow Islamist president Mohamed Morsi, former British prime minister Tony Blair said on Sunday.

Writing in the Observer newspaper, Blair, who acts as Middle East envoy for the United States, Russia, the EU and the U.N., said the army's only alternative would have been to let Egypt descend into chaos.

Russian President Vladimir Putin warned on Sunday that the standoff in Egypt between supporters and foes of deposed president Mohamed Morsi threatened to degenerate into a civil war.

"Syria is already in the grips of a civil war, unfortunately enough, and Egypt is moving in that direction," Russian news agencies quoted Putin as saying while on a visit to Kazakhstan.