Lawyers for Egypt's ex-President Hosni Mubarak entered a not guilty plea when his retrial for alleged complicity in the killings of protesters in 2011 resumed on Saturday.

The hearing comes coincidentally just three days after Mubarak's successor Mohamed Morsi was himself toppled and amid turmoil on the streets pitting Islamists against anti-Morsi protesters.

Syrian warplanes launched a series of strikes on the outskirts of Damascus Saturday as President Bashar Assad's regime pressed a bid to drive back rebels, a monitoring group said.

Fresh fighting meanwhile erupted in several flashpoint areas around the capital, while the army renewed its shelling on besieged rebel areas of the central city of Homs, said the Syrian Observatory for Human Rights.

A bomb hidden in a plastic bag exploded at a checkpoint in the Yemeni capital Sanaa on Saturday, killing three policemen and wounding a fourth, a security services official said.

The device was planted near a police checkpoint in the north of the capital and detonated as the policemen opened the bag to check its contents, the official added.

Yemen has asked the International Organization for Migration to help some 200,000 Yemenis forced to leave Saudi Arabia in the past three months amid a crackdown on undocumented migrants, the IOM said Friday.

Yemen wants the IOM to help provide basic assistance such as shelter, access to water and hygiene kits to vulnerable returnees, the Geneva-based organisation said in a statement.

Islamists massed Saturday in a new show of force to demand the army restore Egypt's first democratically elected leader, after 24 hours of ferocious violence killed 37 people and injured more than 1,400.

Tears flowed freely as thousands of Muslim Brotherhood supporters mourned four members of the movement killed in the violence that erupted during protests against the military's ouster on Wednesday of Islamist president Mohammed Morsi.

A bombing against a Shiite hall in Baghdad killed 15 people Friday, among several attacks to strike Iraq with unrest at its worst since 2008 amid fears of a revival of all-out conflict.

Nationwide violence left 23 dead and dozens more wounded as bombings struck town squares and a husseiniyah, or Shiite religious hall, as the country grapples with a political deadlock and months-long protests by its Sunni Arab minority.

Unidentified gunmen killed two Egyptian policemen guarding a government building in the northern Sinai town of al-Arish on Friday, a security official told Agence France Presse.

The gunmen, who were on a motorbike, shot the policemen dead before fleeing the scene, the official said.

Muslim Brotherhood supreme guide Mohammed Badie told a crowd of supporters of Egypt's ousted president Friday that protesters will remain mobilized until Mohammed Morsi's return after he was deposed by the military.

"Millions will remain in the squares until we carry our elected president, Mohammed Morsi, on our shoulders," Badie told the cheering crowd.

Turkey's prime minister on Friday condemned the military intervention that toppled Egypt's Islamist president Mohamed Morsi as an enemy of democracy, and chastised the West for failing to brand the ouster a coup.

Referring to his country's history of coups, Recep Tayyip Erdogan warned that such military uprisings come at a heavy price and must not be tolerated.

Ismail Haniya, head of the Hamas government in Gaza, appealed in a Friday sermon to have faith in the Arab Spring despite the ouster of Egypt's Islamist president Mohamed Morsi by the army.

"Do not fear for the Palestinian cause or for the resistance (against Israel) or for Gaza. Egypt is behind us, as are the Arab and Islamic countries," Haniya said.