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Egyptian security forces firing tear gas quashed small, scattered demonstrations on Thursday by Islamist supporters of ousted President Mohammed Morsi who tried to commemorate the anniversary of the killing of hundreds of protesters.

After the military overthrew Morsi last summer amid massive demonstrations against his divisive yearlong rule, authorities waged a sweeping crackdown on his Muslim Brotherhood movement and other Islamists, with hundreds killed in street clashes and tens of thousands detained.

The deadliest such incident was exactly one year ago, when security forces dispersed two large pro-Morsi sit-ins in the Cairo intersections of Rabaah el-Adawiya and Nahda, setting off clashes and killing hundreds of demonstrators in the worst mass killing in modern Egyptian history.

Human Rights Watch released a lengthy report this week documenting the dispersal, saying security forces likely committed crimes against humanity and comparing the incident to China's Tiananmen Square massacre. The rights group called for a U.N. investigation into the role of President Abdel-Fattah el-Sissi -- who was military chief at the time -- and other officials in the killings.

Authorities have blamed the Brotherhood for a string of attacks on security forces and have branded it a terrorist organization. The group denies the allegations, insisting it is committed to using protests and other peaceful means to bring about the reinstatement of Morsi, who has been jailed along with virtually the entire top leadership of the movement.

Pro-Morsi demonstrations waned in recent months as security forces have moved aggressively to break up public gatherings. El-Sissi was elected in May by a large majority in a vote boycotted by Morsi's supporters.

In Egypt's second largest city of Alexandria, a one-time Islamist stronghold, security forces used tear gas on Thursday to disperse three small protests after demonstrators set tires alight and blocked main roads while chanting against the military and raising the four-finger sign of Rabaah, which means fourth in Arabic.

A security official said assailants set fire to electrical facilities in the Nile Delta town of Abu Hamad. He spoke on condition of anonymity because he was not authorized to speak to the press. Authorities have accused Morsi supporters of attacking the country's infrastructure, including power facilities and communication towers, and of planting bombs in subway trains.

Most of the deadly attacks over the past year have targeted security forces, with more radical Islamic groups claiming responsibility.