A female demonstrator was killed in clashes with Egyptian police during a rare leftwing protest in central Cairo Saturday, the eve of the anniversary of the 2011 uprising against Hosni Mubarak, an official said.
Shaima al-Sabbagh, who friends said was 34 and the mother of a five-year-old boy, died of birdshot wounds, a health ministry spokesman said.
Fellow protesters said she was hit by birdshot when police fired to disperse the march.
Prime Minister Ibrahim Mahlab said Sabbagh's death was being investigated and vowed that "whoever committed a mistake will be punished, whoever he may be."
A senior interior ministry official denied police had used birdshot to disperse the protest.
"No weapons such as birdshot or rubber bullets were used, it was a small protest that did not require the use of such weapons," an aide to the interior minister, Abdel Fattah Osman, told AFP. "Only two tear gas canisters were fired."
The clash took place hours before state television aired a pre-recorded speech by President Abdel Fattah al-Sisi to mark the fourth anniversary of the popular uprising.
"I salute all our martyrs, from the beginning of January 25 (2011) until now," said Sisi.
The speech appears to have been taped in the presidential palace before Sisi left for Saudi Arabia to offer his condolences over the death of King Abdullah.
Islamists have called for protests on Sunday in a bid to revive the "revolution" that overthrew Mubarak and briefly brought to power Islamist president Mohamed Morsi, toppled by then army chief Sisi in July 2013.
Morsi's supporters often hold small rallies that police quickly disperse.
An 18-year-old female protester had been killed on Friday in clashes in the Mediterranean city of Alexandria.
Police have warned they would confront protests "decisively."
Authorities have cracked down on the Islamists since the military overthrew Morsi after a year in power, and hundreds have been killed in clashes.
Scores of policemen and soldiers have also been killed in militant attacks.
The crackdown has also extended to leftwing and secular dissidents who initially supported Morsi's overthrow but have since turned against the new authorities, accusing them of being authoritarian.
Authorities in Egypt closed off the Tahrir Square area on Saturday night ahead of Sunday's anniversary and increased security presence in other areas of the capital as well.
State news agency MENA said 22 armored vehicles were parked around Tahrir Square and roads to the square were sealed off. Security forces were also dispatched to Rabaa Square in northeast Cairo, where hundreds of supporters of Islamist President Mohamed Morsi were killed in August 2014, one month after the army toppled him.
Although a security crackdown has virtually ended street demonstrations, several took place this week in Cairo and Egypt's second city, Alexandria.
Increased security presence was also felt in Suez, Port Said and Minya due to fear of rioting and violent clashes.
The main concern is of attempts by the Muslim Brotherhood to incite to violence and of terror attacks.
Ceremonies commemorating the January 25 uprising, however, have been postponed after Egypt declared a seven-day mourning period following the death of Saudi King Abdullah.
Saturday's central Cairo protest was organized by the Socialist Popular Alliance party.
"The party decided to hold a symbolic protest to commemorate the anniversary of the January 25 revolution," said member Adel el-Meligy.
Police "fired tear gas, birdshot and arrested the party's secretary general and five other young members," he told AFP.
Sabbagh, a member of the party, was hit in the head with birdshot, and was taken to a hospital where she was declared dead.
The interior ministry suggested in a statement that Islamist "infiltrators" were to blame.
The 18-day anti-Mubarak revolt was fuelled by police abuses and the corruption of the strongman's three-decade rule, but the police have since regained popularity amid widespread yearning for stability.
Activists, including those who spearheaded the anti-Mubarak revolt, have accused Sisi of reviving aspects of the former autocrat's rule.
Sisi and his supporters deny such allegations, and point to his widespread popularity and support for a firm hand in dealing with protests, which are seen as jeopardizing an economic recovery.
The uprising's anniversary comes just days after a court ordered the release of Mubarak's two sons, Gamal and Alaa, pending a corruption retrial along with their father.
Another court had dismissed charges against the elder Mubarak over the deaths of protesters during the 18-day uprising against him.
Roi Kais and Reuters contributed to this report.