By Tom Perry and Maggie Fick
CAIRO (Reuters) - Supporters of deposed President Mohamed Mursi and the army that toppled him prepared for rival protests on Friday as the prime minister warned of violence, with the country more polarized than at any time since Hosni Mubarak was overthrown in early 2011.
Army chief General Abdel Fattah al-Sisi has called on Egyptians to take to the streets to show their support for action against "violence and terrorism". Mursi's Muslim Brotherhood, organizing its own marches on Friday against Sisi, fears the army appeal is a harbinger of a wider crackdown.
Prime Minister Hazem el-Beblawy, head of the interim cabinet, said there were escalating attacks on government institutions by increasingly well-armed protesters.
"The presence of weapons, intimidation, fear - this causes concern, especially when there are calls for many to come out tomorrow from different sides," he told a news conference.
After a month in which more than 100 people, most of them Mursi supporters, have died in violence triggered by his downfall, many fear the protests will lead to more bloodshed.
One security official forecast violence beginning Friday night and stretching into Saturday. "The history of Egypt will be written on those days," said the official, part of a security establishment that accuses the Islamists of turning to violence.
Reiterating his group's commitment to peaceful protest, senior Brotherhood politician Farid Ismail accused the security services of readying militias to attack Mursi supporters, adding that Sisi aimed to drag Egypt into civil war.
"His definition of terrorism is anyone who disagrees with him," Ismail told Reuters. "We are moving forward in complete peacefulness, going forward to confront this coup."
Sisi's speech on Wednesday pointed to the deepening confrontation between the Brotherhood and the military establishment, which has reasserted its role at the heart of government even as it says it aims to steer clear of politics.
Saying it moved against Mursi in response to the biggest popular protests in Egypt's history, the army installed an interim government that set out a plan to hold parliamentary elections in about six months, to be followed by a presidential vote.
The Brotherhood, whose supporters have been camped out in a Cairo suburb for a month, says it wants nothing to do with the transition plan. With Mursi still in military detention at an undisclosed location, there is slim hope for compromise.
The country remains deeply divided about what happened on July 3. The Brotherhood accuses the army of ejecting a democratically elected leader in a long-planned coup, while its opponents say the army responded to the will of the people.
The Brotherhood's political opponents have swung behind the army's call for rallies. "There are men carrying guns on the street ... We will not let extremists ruin our revolution," said Mohammed Abdul Aziz, a spokesman for Tamarud, the anti-Mursi petition campaign that mobilized the protests against his rule.
"Tomorrow we will clean Egypt," he told Reuters.
Sisi announced the nationwide rallies after a bomb attack on a police station in Mansoura, a city north of Cairo in which a policeman was killed. The government said it was a terrorist attack. The Brotherhood also condemned the bombing, accusing the establishment of seeking to frame it to justify a crackdown.
Since Mursi was deposed, hardline Islamist groups have also escalated a violent campaign against the state in the lawless Sinai Peninsula, with daily attacks on the police and army.
The army says Sisi's speech did not represent a threat to any party, reiterating the military's backing for national reconciliation. The army spokesman said any violence at the forthcoming rallies would be dealt with decisively.
At the Brotherhood protest camp in front of a Cairo mosque, some Mursi supporters forecast trouble ahead.
"The army itself will strike. They will use thugs and the police," said Sarah Ahmad, a 24-year-old medical student.
Essam El-Erian, another senior Brotherhood politician, accused "the putschists" of trying to recreate a police state. "This state will never return, and Egypt will not go backwards," he said in a televised news conference.
(Additional reporting by Asma Alsharif, Yasmine Saleh, Noah Browning, Tom Finn and Shadia Nasralla; Writing by Tom Perry; Editing by Crispian Balmer and Will Waterman)