* Violence reported in several cities after Friday prayers

* Armoured vehicles deployed across Cairo

* Reuters witness counts 27 bodies in Cairo mosque

* Saudi Arabia stands with Egypt against "terrorism" - King

By Tom Perry and Alexander Dziadosz

CAIRO, Aug 16 (Reuters) - Muslim Brotherhood protests plunged into violence across Egypt on Friday, with around 50 killed in Cairo alone on a "Day of Rage" called by Islamist followers of ousted President Mohamed Mursi to denounce a police crackdown.

Automatic gunfire echoed across Cairo and black smoke billowed from the capital's huge Ramses Square, a military helicopter hovering low overhead looking down on the chaos.

A Reuters witness saw the bodies of 27 people, apparently hit by gunfire and birdshot, wrapped in white sheets in a mosque. A Reuters photographer said security forces opened fire from numerous directions when a police station was attacked.

At least 20 people died in clashes elsewhere in Egypt.

The violence followed Wednesday's assault by security forces on two Brotherhood sit-ins in Cairo that left hundreds dead, as the military-backed government tried to end weeks of turbulence that has pushed the Arab world's most populous state to the brink of disaster.

Western governments urged restraint and Germany cautioned the new government that it was reviewing its ties. By contrast, Saudi Arabia's King Abdullah said his country stood with Egypt in its battle against "terrorism".

The army deployed armoured vehicles on major roads around the capital and the Interior Ministry said police would use live ammunition against anyone threatening public buildings.

"Sooner or later I will die. Better to die for my rights than in my bed. Guns don't scare us anymore," said Sara Ahmed, 28, a business manager who joined the demonstrators in Cairo.

"It's not about the Brotherhood, it's about human rights," said Ahmed, one of the few women in the crowd not wearing a headscarf, a sign of piety for Muslim women.

Anger on the streets was directed at army commander General Abdel Fattah al-Sisi, who moved against Mursi last month after massive street rallies against the his administration that had been dogged by accusations of incompetence and partisanship.

"The people want the butcher executed," said Mustafa Ibrahim, 37, referring to Sisi, as he marched with a crowd of several thousand on downtown Cairo under blazing summer sun.

DEATH

Emergency services said eight protesters were killed in clashes in the Mediterranean town of Damietta, five in Fayoum south of Cairo, four in the Suez Canal city of Ismailia and four in the Nile delta town of Tanta. One person was killed in Alexandria, Egypt's second city.

A police conscript was killed in a drive-by shooting in the north of the capital, state news agency MENA reported. Nile TV showed footage of a gunman among Islamist protesters firing from a central Cairo bridge.

Signalling his displeasure at the worst bloodshed in Egypt for generations, U.S. President Barack Obama said on Thursday normal cooperation with Cairo could not continue and announced the cancellation of military exercises with Egypt next month.

"We deplore violence against civilians. We support universal rights essential to human dignity, including the right to peaceful protest," he said, but stopped short of cutting off the $1.55 billion a year of mostly military U.S. aid to Egypt.

The Brotherhood accuses the military of staging a coup when it ousted Mursi on July 3. Liberal and youth activists who backed the military saw the move as a positive response to public demands.

But some fear Egypt is turning back into the kind of police state that kept the disgraced Hosni Mubarak in power for 30 years before his removal in 2011, as security institutions recover their confidence and reassert control.

In calling for a "Day of Rage," the Brotherhood used the same name as that given to the most violent day of the uprising against Mubarak. That day, Jan. 28, 2011, marked the protesters' victory over the police, who were forced to retreat.

Ironically, the centre of the anti-Mubarak protests, Tahrir Square, was deserted on Friday, sealed off by the army.

SAUDI SUPPORT

Underscoring the deep divisions in the country, local residents helped the army block access to Cairo's Rabaa al-Adawiya mosque, the site of the main Brotherhood sit-in that was swept away during Wednesday's police assault.

"We are here to prevent those filthy bastards from coming back," said Mohamed Ali, a 22-year-old business student.

The Egyptian presidency issued a statement criticising Obama, saying his comments were not based on "facts" and would strengthen violent groups that were committing "terrorist acts".

Pro-army groups posted videos on the Internet of policemen they said had been tortured and killed by Islamist militants.

Washington's influence over Cairo has been called into question following Mursi's overthrow. Since then Saudi Arabia, Kuwait and the United Arab Emirates have pledged $12 billion to Egypt, making them more prominent partners.

"The Kingdom of Saudi Arabia, its people and government stood and stand by today with its brothers in Egypt against terrorism," King Abdullah said in an uncompromising message read out on Saudi television.

"I call on the honest men of Egypt and the Arab and Muslim nations ... to stand as one man and with one heart in the face of attempts to destabilise a country that is at the forefront of Arab and Muslim history," he added.

Obama's refusal so far to cut off U.S. aid to Egypt suggests he does not wish to alienate the generals, despite the scale of the bloodshed in the army's suppression of Mursi supporters.

Egypt will need all the financial support it can get in the coming months as it grapples with growing economic problems, especially in the important tourism sector that accounts for more than 10 percent of gross domestic product.

The United States urged its citizens to leave Egypt on Thursday and two of Europe's biggest tour operators, Germany's TUI and Thomas Cook Germany, said they were cancelling all trips to the country until Sept. 15.

When a military helicopter flew low over Ramses Square, protesters held up shoes chanting "We will bring Sisi to the ground" and "Leave, leave, you traitor".

As the sound of teargas canisters being fired began, protesters - including young and old, men and women - donned surgical masks, gas masks and wrapped bandannas around their faces. Some rubbed Pepsi on their faces to counter the gas.

"Allahu akbar! (God is Greatest)" the crowd chanted.