At least seven people have been killed and 261 injured in overnight clashes between security forces and supporters of Egypt's toppled president Mohammed Morsi, authorities said.

Violence broke out as thousands of pro-Muslim Brotherhood demonstrators took to the streets of Cairo demanding Morsi be reinstated as president, only to be confronted by opponents and police.

Fighting erupted in different locations in Cairo and carried on into the early morning hours according to Khaled el-Khateib, a senior Health Ministry official.

The head of Egypt's emergency services Mohamed Sultan told Reuters two people were killed at a bridge in central Cairo.

Police fired teargas at demonstrators blocking the central Sixth of October Bridge near Ramsis square, who responded hurling stones.

"We were crouched on the ground, we were praying. Suddenly there was shouting. We looked up and the police were on the bridge firing tear gas down on us," pro-Mursi protester Adel Asman, 42, told Reuters.

The Muslim Brotherhood has accused security forces of using birdshot and live ammunition.

The other deaths reportedly occurred in clashes at a protest sit-in near a Cairo university campus.

Clashes were also reported in other parts of the Egyptian capital, while protest rallies were staged also in the coastal city of Alexandria and the Nile city of Assiut.

Political violence has gripped the country since the military deposed Morsi after days of mass street protests, earlier this month.

Some 51 people were killed as security forces protecting the compound, where the former president was reportedly being held, opened fire on demonstrators last week.

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