Egyptian army chief Abdel Fattah al-Sisi said Wednesday he had quit the military to run for president and vowed to rid the country of "terrorism" almost nine months after he toppled its elected leader.

Sisi, who was also defence minister, faces no serious competition in the election -- likely before June -- and is expected to win comfortably, riding on a wave of popularity for his law and order message.

But the Muslim Brotherhood movement of president Mohamed Morsi, whom Sisi toppled in July, warned there could be no stability in Egypt under the "shadow" of his leadership.

The Brotherhood and its allies have kept up their protests against the overthrow of Egypt's only freely elected president against a backdrop of mounting violence by militant groups that has killed scores of police and troops.

Sisi declared his widely anticipated candidacy in a televised address to the nation.

"Today, I stand before you for the last time in a military uniform, after deciding to end my service as defence minister and commander of the armed forces," he said, dressed in his field marshal's uniform and sitting behind a desk.

"With all modesty, I nominate myself for the presidency of Egypt."

Sisi's candidacy is likely to be welcomed by the millions of Egyptians who are weary of more than three years of turmoil since the Arab Spring overthrow of veteran strongman Hosni Mubarak.

But it is likely to further inflame Islamist protests and worry those secular activists who fear a return to rule by military men and the strong-arm tactics of the Mubarak era.

Aside from Morsi, whose year in office deeply polarised the country, every Egyptian president has been drawn from, or installed by, the army.

For those Egyptians who want an end to the unrest that has scared off investors and tourists, dealing a heavy blow to the economy, Sisi's military background is an asset.

The army is seen as the country's most stable institution and Sisi can count on further aid from friendly Gulf states, who have pumped billions of dollars into Egypt since Morsi's ouster.

- 'Egypt free of terrorism' -

"The economic, political, social and security realities in Egypt... have reached the point where they must be tackled with strength and courage," Sisi said.

"I will continue to fight every day for an Egypt free of terrorism."

Sisi reached out a little to his opponents but signalled no let-up in the crackdown launched by the military-installed interim government on Morsi's supporters that has seen hundreds killed and thousands arrested.

"All Egyptians who have not been condemned by the law... will be active partners in the future," he said.

Sisi's suitability for the presidency was rejected outright by the Muslim Brotherhood.

"He led a coup to become president. He is a man who has killed daily since the coup," Ibrahim Munir, a member of the Brotherhood's political bureau, told AFP by telephone from London.

Just hours before Sisi's address, clashes between Morsi's student supporters and police killed one protester at a Cairo campus.

In southern Egypt, prosecutors referred more than 900 suspected Islamists to trial, two days after a court sentenced 529 to death for deadly rioting.

US Secretary of State John Kerry, in a statement issued while visiting Jordan, urged Egypt to overturn those death sentences, warning they sent a "negative message" around the world.

Kerry said he was "deeply troubled by the sudden and unprecedented decision" to issue the sentences, adding "it simply defies logic".

Morsi himself, detained on his ouster on July 3, faces several trials along with much of the Muslim Brotherhood leadership.

Sisi is believed to have been the real power behind interim president Adly Mansour, under whom police killed hundreds of Islamist protesters and detained about 15,000 suspects.

Many Egyptians, deeply disillusioned by the Islamist Morsi's single year in power, have supported the crackdown in the hope of stability. Millions had rallied across the country demanding Morsi's overthrow before the military stepped in.

In Cairo's Tahrir Square, the nerve centre of the 2011 uprising that unseated Mubarak, a passerby said he was delighted with Sisi's candidacy.

"We need a man like him for the country now," said Ahmed Ali, an engineer.