Ousted president Mohammed Morsi was defiant as he went on trial Tuesday for a prison break during the 2011 uprising, as a top police official was murdered in another sign of Egypt's instability.

The trial, and other violence in which a police guard was gunned down outside a Cairo church, came a day after the military backed army chief Field Marshall Abdel Fattah al-Sisi, who led Morsi's ouster in July, to run for office in his place.

At least 29 people were killed in clashes in Egypt Saturday during rival rallies on the anniversary of the 2011 revolt that toppled Hosni Mubarak, underscoring the country's violent polarization.

Three years after Egyptians rose up to demand the overthrow of Mubarak, thousands of demonstrators in Cairo's Tahrir Square chanted slogans backing another military man, General Abdel Fattah al-Sisi, as police clashed with Islamists and activists elsewhere.

An assailant threw a small bomb at a Cairo police building Saturday, police said, as rival demonstrators prepared to commemorate the 2011 uprising and a day after a spate of deadly bombings.

The small "incendiary bomb" lodged on the wall of the police training center and exploded without causing any casualties, a police official said.

A suicide car bomber struck Cairo police headquarters Friday, the first of four blasts in Egypt's capital that killed six people on the eve of the anniversary of the 2011 uprising.

The bombings, all targeting the police, came as street clashes between Islamist supporters of deposed president Mohammed Morsi and backers of the military killed eight people, a day before planned rallies to mark the revolt that drove Hosni Mubarak from power.

Masked gunmen on motorbikes killed five Egyptian policemen when they opened fire on a checkpoint south of Cairo on Thursday, the interior ministry said.

The ministry said two policemen were also wounded in the attack in the province of Beni Suef, which came two days ahead of the commemoration of the third anniversary of the 2011 uprising.

A Cairo court jailed three people for two years Wednesday for destroying a monument to those killed in mass protests that unseated two Egyptian presidents since 2011, judicial sources said.

The monument was built by the military-installed authorities in Cairo's Tahrir Square -- epicenter of the revolt that ousted long-time ruler Hosni Mubarak in early 2011.

A crackdown by Egypt's authorities on the Muslim Brotherhood has left hundreds of Islamic charities struggling for funds, with experts warning the move could impact this week's referendum on a new constitution.

The Tuesday-Wednesday vote has been billed as the first step in Egypt's democratic transition after the military overthrew Islamist president Mohammed Morsi in July following huge protests against his one-year rule.

An Egyptian court on Wednesday adjourned the murder trial of deposed President Mohammed Morsi to February 1, citing "weather conditions" that prevented the Islamist's transport to court from prison.

It had been scheduled as the second hearing in Morsi's trial, after an initial court appearance in November in which he denounced the tribunal and insisted he was still the country's president.

Egypt's interim president on Sunday made a rare visit to see the pontiff of the nation's Orthodox Christians at St. Mark's Cathedral, the papal seat in central Cairo, according to a brief statement by the church.

The highly symbolic visit to Pope Tawadros II by Adly Mansour was made ahead of the Coptic Christmas, which falls on Tuesday.

At least 13 people were killed Friday in clashes across Egypt as police dispersed thousands of protesters demanding the reinstatement of deposed Islamist president Mohammed Morsi, the health ministry said.

The protests came after an Islamist alliance that backs Morsi called for demonstrations ahead of a new hearing on Wednesday in a trial of the ousted president.