The Cairo campaign headquarters of Egyptian presidential candidate Ahmed Shafiq has been attacked.
Egyptian TV broadcast footage of a fire at the building, in the Dokki district. The fire was put out with no signs of serious damage, and no injuries.
The attack came hours after it was announced that Mr Shafiq - The last PM of ex-President Hosni Mubarak - would compete in a run-off next month.
He will face Muslim Brotherhood's Mohammed Mursi in the election.
Eight suspects were arrested near Mr Shafiq's headquarters, according to police.
The attackers also took a number of campaign posters and allegedly took a number of computers from the building, the BBC's Jon Leyne reports from the headquarters.
Anti-Shafiq protesters then dispersed and a number of Mr Shafiq's supporters then arrived at the scene, furious at the attack, our correspondent adds.
No-one knows who carried out the attack. Mr Shafiq is the candidate of law and order, so perversely, the more trouble there is, the more he could benefit, he adds.
Crowds also headed to the city's central Tahrir Square, scene of the protests which forced the resignation of President Hosni Mubarak in February 2011 and frequent demonstrations ever since.
Several hundred people, perhaps as many as a thousand gathered in the roundabout at the centre of the square, protesting against the official election results, the BBC's Yolande Knell reports from the square.
Protests were also reported in Egypt's second city of Alexandria, where left-wing candidate Hamdeen Sabahi topped the poll in the first round of voting last week.
Earlier on Monday, Egypt's election commission confirmed that Mr Mursi had gained 24.3% of the vote in the first round, while Mr Shafiq won 23.3%.
However, there is real anger at the results among many activists, our correspondent says.
Many are saying that they are prepared to stay in the square and that this is the "last chance" to save Egypt's revolution.
Mr Shafiq is viewed by many as a representative of the old regime.