(Updates protests, adds Obama)
* Only small numbers appear for post-prayers protests
* Islamist movement reeling from bloodshed, arrests
* Obama says re-evaluating ties with Egypt
* Trial of its top three leaders to open on Sunday
By Maggie Fick and Lin Noueihed
CAIRO, Aug 23 (Reuters) - Mass protests called by Egypt's Muslim Brotherhood mostly failed to materialise on Friday as the movement reeled from a bloody army crackdown on followers of ousted President Mohamed Mursi.
Troops and police had taken relatively low-key security measures before the "Friday of Martyrs" processions that were to have begun from 28 mosques in the capital after weekly prayers.
But midday prayers were cancelled at some mosques and there were few signs of major demonstrations unfolding in Cairo.
"We are not afraid; it's victory or death," said Mohamed Abdel Azim, a retired oil engineer who was among about 100 people marching slowly from a mosque near Cairo University.
"They intend to strike at Muslims," the grey-bearded Azim said. "We'd rather die in dignity than live in oppression. We'll keep coming out until there's no one left."
Some marchers carried posters of Mursi, who was toppled by army chief General Abdel Fattah al-Sisi on July 3 after huge demonstrations against his rule. "No to the coup," they chanted.
Egypt has endured the bloodiest civil unrest in its modern history since Aug. 14 when police destroyed protest camps set up by Mursi's supporters in Cairo to demand his reinstatement.
The violence has alarmed Egypt's Western allies, but U.S. President Barack Obama acknowledged that even a decision to cut off U.S. aid to Cairo might not influence its military rulers.
Some U.S. lawmakers have called for a halt to the $1.5 billion a year in mostly military assistance to Egypt.
"The aid itself may not reverse what the interim government does," Obama said in an interview with CNN. "But I think what most Americans would say is that we have to be very careful about being seen as aiding and abetting actions that we think run contrary to our values and our ideals."
He said the United States was re-evaluating its ties with Egypt. "There's no doubt that we can't return to business as usual, given what's happened," he said.
The United States has nurtured an alliance with Egypt since it signed a peace treaty with Israel in 1979. Military cooperation includes privileged U.S. access to the Suez Canal.
The Brotherhood, hounded by Egypt's new army-backed rulers, had called for demonstrations across the country against the crackdown, testing the resilience of its battered support base.
A few dozen Islamists, many of them women, marched in an old Cairo district. Some carried Egyptian flags or rolled-up Mursi posters. Others held umbrellas to ward off the afternoon sun.
Asked if she was afraid, a fully veiled nursery teacher with four children, who gave her name only as Nasra, said: "God will make us victorious even if many of us are hurt and even if it takes a long time. God willing, God will bring down Sisi."
Security forces kept a watchful eye, but did not flood the streets, even near Cairo's central Fateh mosque where gun battles killed scores of people last Friday and Saturday.
The mosque's metal gates and big front door were locked and chained. Prayers were cancelled. Two armoured vehicles were parked down the street, where people shopped at a busy market.
Only one riot police truck stood by near Rabaa al-Adawiya square in northeastern Cairo, home to the Brotherhood's biggest protest vigil until police and troops stormed in, killing hundreds of people, bulldozing barricades and burning tents.
The mosque there was closed for repairs. Workmen in blue overalls stood on scaffolding as they covered its charred walls with white paint. Children scavenged through piles of garbage.
Troops used barbed wire to block a main road to Nahda Square, the site of the smaller of the two Brotherhood sit-ins.
The authorities declared a month-long state of emergency last week and they enforce a nightly curfew. The state news agency said the armed forces had strengthened their presence around the presidential palace and the defence ministry.
The Muslim Brotherhood, which won five successive votes held in Egypt after the overthrow of Hosni Mubarak in 2011, has been jolted by a week of bloodshed and the arrest of many of its leaders in what the authorities call a battle with terrorism.
In a symbolic victory for the army-dominated old order, Hosni Mubarak, the ex-military former president who ruled Egypt with an iron fist for 30 years before an uprising toppled him in 2011, was freed from jail on Thursday. His successor Mursi, Egypt's first freely elected president, remains behind bars.
The Brotherhood's "General Guide" Mohamed Badie, who was arrested on Tuesday, is due to go on trial on Sunday along with two other senior figures, Khairat al-Shater and Saad al-Katatni, on charges that include incitement to violence.
At least 900 people, including about 100 soldiers and police, have been killed in violence across Egypt since Aug. 14, officials say. Brotherhood supporters say the toll is higher.
The Brotherhood, founded in 1928, could once mobilise vast crowds, drawing on the same organisational strength it wielded at the ballot box, but its protests have dwindled this week in the face of the security crackdown.
Graffiti on a mosque wall in a run-down Cairo neighbourhood illustrated the deep divisions that have emerged since Sisi ousted Mursi. The spray-painted message "Yes to Sisi" had been crossed out and painted over with the word "traitor."
Slogans elsewhere read "Mursi is a spy" and "Mursi out". Someone had also written "Freedom, Justice, Brotherhood".
The Brotherhood survived for generations as an underground movement before emerging as Egypt's best-drilled political force after Mubarak fell. But its popularity waned during Mursi's year in office when critics accused it of accumulating power, pushing a partisan Islamist agenda and mismanaging the economy.
The Brotherhood, which Egypt's new army-backed rulers have threatened to dissolve, says Mursi's government was deliberately undermined by unreconstructed Mubarak-era institutions.
Mubarak, 85, still faces retrial on charges of complicity in the killings of protesters, but he left jail on Thursday for the first time since April 2011 and was flown by helicopter to a plush military hospital in the southern Cairo suburb of Maadi.
The authorities have used the state of emergency to keep him under house arrest, apparently to minimise the risk of popular anger if he had been given unfettered freedom. Local newspapers on Friday focused on the latest arrest of Brotherhood leaders, giving scant coverage to the former strongman's exit from jail. (Additional reporting by Cairo bureau; Writing by Alistair Lyon; Editing by Will Waterman and Alastair Macdonald)