* Protesters put death toll at 30 as police clear Cairo camps
* Muslim Brotherhood may lose main source of leverage
* Political, security crisis has divided society (adds latest witness accounts, details, background)
Egyptian security forces killed at least 30 people on Wednesday when they moved in to clear a camp of Cairo protesters demanding the reinstatement of deposed President Mohamed Mursi, his Muslim Brotherhood movement said.
There was no official confirmation of the death toll at Rabaa al-Adawiya, in northeast Cairo, where thousands of Mursi supporters have staged a six-week sit-in that caused the army acute embarrassment since it ousted the elected leader.
A second camp near Cairo University was swiftly cleared in the early morning.
The operation, which suggested that the powerful military had lost patience with persistent protests that were crippling parts of the capital and slowing the political process, began just after dawn with helicopters hovering over the camps.
Gunfire rang out as protesters, among them women and children, fled Rabaa, and clouds of black smoke rose into the air. Armoured vehicles moved in beside bulldozers which began clearing tents. One witness saw 15 bodies at a field hospital.
The Health Ministry said 13 people were killed near Rabaa during the crackdown, including five police and eight civilians. The official death toll could well rise.
The government issued a statement saying security forces had showed the "utmost degree of self-restraint", reflected in low casualties compared to the number of people "and the volume of weapons and violence directed against the security forces".
"Tear gas (canisters) were falling from the sky like rain. There are no ambulances inside. They closed every entrance," said protester Khaled Ahmed, 20, a university student wearing a hard hat with tears streaming down his face.
"There are women and children in there. God help them. This is a siege, a military attack on a civilian protest camp."
"At 7 a.m. they came. Helicopters from the top and bulldozers from below. They smashed through our walls. Police and soldiers, they fired tear gas at children," said teacher Saleh Abdulaziz, 39, clutching a bleeding wound on his head.
"They continued to fire at protesters even when we begged them to stop."
The operation came after international efforts failed to mediate an end to the political standoff between Mursi's supporters and the army-backed government which took power after his ouster on July 3.
With the Brotherhood calling on supporters to take to the streets, the violence risked further destabilising the most populous Arab nation and endangering hopes for democracy.
More than 300 people have already died in political violence since Mursi's overthrow, including dozens of supporters killed by security forces in two separate earlier incidents in Cairo.
The unrest spread beyond the capital on Wednesday, with the Nile Delta cities of Minya and Assiut also rocked by violence.
Security forces fired tear gas at thousands of Mursi supporters who had set part of a church on fire in Minya. In Assiut, about 3,000 pro-Mursi protesters clashed with police.
In the coastal town of Marsa Matruh, police fired tear gas to break up hundreds of stone-throwing protesters in front of the governorate headquarters.
Egypt has been convulsed by political and economic turmoil since a 2011 uprising that ended 30 years of autocratic rule by U.S.-backed President Hosni Mubarak, and the country is now more polarised than any time for many years.
Mursi became Egypt's first freely elected leader in June 2012, but failed to tackle deep economic malaise and worried many Egyptians with apparent efforts to tighten Islamist rule.
Liberals and young Egyptians staged huge rallies demanding that he resign, and the army said it removed him in response to the will of the people.
Mursi's Brotherhood movement, suppressed for decades under Mubarak, staged sit-in protests and mass marches across Egypt in response, and said they would continue until the deposed leader was reinstated.
Wednesday's events indicated that the armed forces, led by General Abdel Fattah al-Sisi, had had enough.
The military has installed a temporary government to implement what it calls a "road map" to democracy, which envisages holding fresh elections in around six months.
But its plans have been overshadowed by dissent and violence in a country that has a peace treaty with close U.S. ally Israel and controls the Suez Canal, a waterway vital for global trade.
The West, notably the United States which gives the Egyptian military $1.3 billion in aid each year, has expressed alarm at the violence, and on Wednesday the European Union urged authorities to show "utmost restraint".
After Mursi's removal, Gulf Arab oil producers promised Egypt aid packages worth $12 billion, throwing the country a lifeline at a time of dwindling reserves and food stocks.
ENDGAME FOR BROTHERHOOD?
The attempt to break up the camps appeared to dash any remaining hopes of bringing the Brotherhood back into the political mainstream, and underlined the impression many Egyptians share that the military is tightening its grip.
On Tuesday, interim President Adli Mansour named at least 18 new provincial governors, half of them retired generals, in a shake-up that pushed out Brotherhood members and restored the influence of men from army and police backgrounds.
Wednesday's crackdown could strip the Brotherhood of its main leverage against the government. Some of the group's leaders have been arrested or are wanted and their assets frozen in one of the toughest crackdowns it has ever faced.
Mursi remains detained in an unknown location.