At least 20 people have been killed in protests held across Egypt as the country marked the fraught anniversary of its 2011 popular uprising.

In demonstrations across the country on Sunday, protesters were killed in Cairo, the El-Matariya district just north of Cairo, Damanhur, Alexandria and in Giza. A security conscript was also killed in the El-Matariya district protest.

Egyptian authorities tightened security in Cairo and other cities in anticipation of the protests. Police sealed off several main squares in Cairo, including Tahrir, and beefed up security at vital state installations.

The stepped-up security came as activists mourned the death of a female protester shot on Saturday in downtown Cairo while taking part in a gathering commemorating the nearly 900 protesters killed in the revolt, and another killed in Alexandria on Friday.

Activists blame police for the death of Shaimaa el-Sabagh, a 32-year-old mother of one from the Mediterranean port city of Alexandria, as well as the death of 17-year-old Sondos Rida Abu Bakr. The government says it is investigating.

Videos posted online show el-Sabagh, a member of the leftist Popular Alliance party, with other protesters carrying placards and chanting "bread, freedom and social justice", a chief slogan of the 2011 uprising.

Protesters near her carried wreaths of roses they intended to place at nearby Tahrir Square, the birthplace of the uprising, in memory of the fallen protesters.

In the videos, gunshots ring out and el-Sabagh falls. She is later shown being carried by her husband as blood seeped out of her mouth.

A coalition opposed to the government of President Abdel Fatah el-Sisi and the military coup said on Sunday that it intended to stage street protests to mark the anniversary.

Sisi's government has shown zero tolerance for street protests since a law adopted in 2013 banned them without prior government permission. Thousands of activists have been convicted and jailed for violating the law.

A parallel crackdown is targeting supporters of ousted President Morsi, with thousands from his Muslim Brotherhood group imprisoned or facing trial.

PHOTO CAPTION

Protesters fill the streets in downtown Cairo, Egypt, 25 January 2015.

Al-Jazeera