* Biggest show of Brotherhood defiance for two weeks
* Heavy security presence, limited violence reported (Updates death and casualty toll, adds end of palace protest)
By Maggie Fick
CAIRO, Aug 30 (Reuters) - Thousands of supporters of ousted Islamist president Mohamed Mursi marched through Cairo and cities across Egypt on Friday to demand his reinstatement, in the movement's biggest show of defiance since hundreds of protesters were killed two weeks ago.
Although most marches passed without major incident, a security source said there had been at least six deaths, and police fired teargas at protesters in Cairo's Mohandiseen district.
The army-backed government, which has shot dead hundreds of supporters of Mursi's Muslim Brotherhood since he was toppled by the military on July 3, had warned that forces posted at key intersections since morning would open fire if protests turned violent.
Having arrested most of the Brotherhood's leaders, it hoped by now to have suffocated the protests against its decision to force out and crush the movement that ruled Egypt for a year.
But its prospects of presenting a return to normality looked to have been set back by live television pictures of teargas and burning tyres in Cairo, as well as the sheer number of separate marches that the well-organised Brotherhood managed to stage.
The security source said there had been at least 50 injured throughout Egypt, in addition to the six dead, and more than 20 arrests. The cabinet issued a statement after the protests saying that anyone who disregarded the curfew would face legal consequences.
The demonstrators appeared mostly to have opted for numerous scattered protests, avoiding Cairo's bigger squares or the scenes of earlier protests such as the pro-Mursi street camps where security forces shot dead more than 600 people on Aug. 14.
Just after Friday prayers, around 500 protesters set off from central Cairo's Sahib Rumi mosque, chanting: "Wake up, don't be afraid, the army must leave", "The Interior Ministry are thugs" and "Egypt is Islamic, not secular".
By mid-afternoon, thousands were marching in districts across Cairo calling for the return of the elected government, and some remained outside the presidential palace in the capital until just before the 7 p.m. (1700 GMT) curfew.
Soldiers and helmeted police in black uniforms and bulletproof vests, armed with teargas and semi-automatic rifles, manned checkpoints near the protests and blocked roads.
In Egypt's second city, Alexandria, a total of more than 10,000 protesters took part in several separate demonstrations.
Marches were also held in several cities in the Nile Delta including Tanta, in the three Suez Canal cities of Suez, Ismailia and Port Said, and in the southern city of Assiut.
The Brotherhood's London press office circulated an email with links to video streams from what it said were protests in 15 districts of Cairo, as well as 32 in other towns and cities.
In the city of Fayoum, the private television channel CBC showed footage of a female Brotherhood supporter in a black head-to-toe veil, leading a march of veiled women and carrying a placard reading "Where did legitimacy go?".
"This revolutionary wave will not stop," Brotherhood politician Farid Ismail said by phone from an undisclosed location.
He said the numbers of those who had demonstrated despite "intimidation, teargas, live bullets and detentions" had been hugely underreported.
"This will continue in the coming weeks," he said.
Armed forces chief General Abdel Fattah al-Sisi and the government he backs appear to have won broad public support for their crackdown, which they portray in the largely state-controlled or pro-government media as a fight against terrorism.
The Brotherhood, which won five popular votes after the overthrow of the military-backed president Hosni Mubarak in 2011, and gave Egypt its first civilian president in Mursi, says it is still committed to peaceful resistance.
For decades Egypt's only effective opposition group, despite being officially banned, the Brotherhood seized on the anti-Mubarak uprising to win the presidency.
But millions were alienated by its ideologically driven rule and failure to revive the economy, and took to the streets, giving the army-backed establishment its cue to act with more venom than ever against a group it had repressed for decades.
From early morning, armoured vehicles fitted with machineguns guarded key points such as Tahrir Square, epicentre of the 2011 uprising, and Ramses Square, where more than 100 people, mostly Mursi supporters, were killed in protests two weeks ago. More than 100 members of the security forces have also been killed in the turmoil.
"Today's big turnout show the Brotherhood's ability to organise itself and proves its structure still stands," political analyst Mustapha Al-Sayyid said.
"This will pose a challenge on the authorities to end the protests and regain stability, or to decrease the curfew hours, which is the first step to bringing life back to normal."
The Egyptian-born Muslim cleric Sheikh Youssef al-Qaradawi earlier exhorted Egyptians to take to the streets.
"You Egyptians, go out, all of you, men, mothers, daughters even children. This is a religious duty on all Egyptians!" he said in a Friday sermon broadcast on Qatari state television.
The crackdown on Islamists has soured relations between Egypt and Qatar, a wealthy Gulf Arab state and U.S. ally that backed the Brotherhood and gave Egypt $7 billion during Mursi's administration. (Additional reporting by Cairo Arabic Desk, Yasmine Saleh, and by Sami Aboudi in Dubai; Writing by Kevin Liffey; Editing by Alison Williams)