The Egyptian Interior Ministry said the protester had been killed by accident, an account backed by Ahmad Zeidan, 18, an activist at the sit-in who said he had seen the youth being run over.
"It wasn't deliberate. They (police) were retreating quickly because (protesters) were throwing rocks and Molotov cocktails at them," he said. The demonstrators had come from Tahrir to confront police vehicles apparently heading for the square.
The turmoil of the past week has overshadowed the election, whose initial stage involves Cairo, Alexandria and other areas.
One declared reason for the polling marathon is because judges, who retain public respect for their independence, will supervise the election and there are not enough of them available for a single day of nationwide voting.
Reflecting security concerns, Ahmed al-Zind, head of Egypt's Judges Club, told a news conference the organization had taken out private insurance to cover all the judges involved. Protesters in Tahrir seemed in two minds about the election.
Egypt protest morning
Emad Mohamed, 35, wearing a hat in Egyptian national colors, had no faith in the vote, saying it would enable Mubarak-era politicians to make a come-back. "We do not think it is in our interest. Where are the new parties?" he asked.
But Yasser Nasr, assisting at a makeshift clinic, said: "They cannot delay elections. It will mess up the situation. Once they happen, hopefully things will improve."
The Muslim Brotherhood and other mainstream parties which have not joined the protesters in Tahrir want the election to go ahead, eager to establish a strong presence in parliament.
They have accepted the army's transition timetable, but the demonstrators are demanding an immediate end to military rule.
Instead, the generals have promised that a new president will be elected by mid-2012, sooner than previously announced.
While tens of thousands packed Tahrir Square for what activists dubbed "Last Chance Friday", at least 5,000 people demonstrated in support of the army in another Cairo square, highlighting splits between youngsters bent on radical reform and people upset by the unrest who want to restore normality.
Ganzouri described his task as thankless and "extremely difficult", saying his priority was to secure the streets and revive the economy. Egypt's pound has hit a seven-year low and foreign reserves have dropped by a third since December 2010.
Egyptian protesters demanding an end to army rule clashed with police firing tear gas in central Cairo on Saturday in a flare-up that cast another shadow over a parliamentary election billed as the nation's first free vote in decades.
Two days of voting begin on Monday in the first stage of a complex, drawn-out election that will be completed in January.
One protester, Ahmed Sayed, 21, died after being hit by a riot police vehicle in the clashes. His death was the first since a truce between police and protesters on Thursday calmed violence that had killed 41 people around Cairo's Tahrir Square and elsewhere.
Alarmed by the violence, the United States and the European Union have urged a swift handover to civilian rule in a country where prolonged political turmoil has compounded economic woes.
The latest clash occurred near the cabinet office on the second day of a sit-in to protest against the army's appointment of 78-year-old Kamal Ganzouri, a premier under ousted President Hosni Mubarak, to form a "national salvation government".
An army source said the ruling military council held separate talks with presidential candidates Mohamed ElBaradei and Amr Moussa. "I met Field Marshal Hussein Tantawi over the current crisis and discussed ways to resolve it," Moussa told Reuters later, but gave no details.
ElBaradei's office said in a statement that he had been in touch with all relevant parties to "activate the demands of the revolution". It said he had met Tantawi and army chief of staff Sami Enan to discuss these demands, without reaching agreement.
Protest groups have named ElBaradei as their choice to head a civilian body to supervise Egypt's transition to democracy instead of the army council that took over from Mubarak.
Ganzouri met with youth activists, but the April 6 movement, prominent in the anti-Mubarak revolt, disavowed those involved, saying they were "planted by the military council".
Ganzouri told reporters he had not yet offered anyone a ministerial portfolio. "I am ready to sit with all political currents. The new government will have new faces and youth will be part of it as they are the backbone of this nation," he said.
Tahrir Square protesters have dismissed Ganzouri, premier from 1996 to 1999, as another face from the past whose appointment reflects the generals' resistance to change.
A television clip circulated on Facebook in the past 24 hours shows him sitting one seat away from Tantawi on Jan. 25, the first day of Egypt's revolt, as they listen to a speech by former Interior Minister Habib al-Adli, who is on trial with Mubarak on charges of ordering protesters to be killed.
"Down, down with the marshal," a group chanted in Tahrir, near tents set up on grassy patches. They were referring to Tantawi, who was Mubarak's defense minister for 20 years.
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