A group of protesters ransacked the campaign headquarters of Egyptian presidential candidate Ahmed Shafiq in Cairo on May 28, before setting the premises ablaze.
There were no reported injuries, and the fire was quickly put out with no signs of serious damage.
A number of Shafiq's supporters later gathered at the scene, chanting their support.
The attack came amid protests across Egypt against election results hours after it was announced that Shafiq would compete in a run-off.
Reports said several hundred people had gathered in Cairo’s central Tahrir Square, scene of the mass demonstrations which forced the resignation of President Hosni Mubarak in February 2011.
Shafiq was Hosni Mubarak's last prime minister and many Egyptians worry that a win by him could lead to a revival of the Mubarak regime.
A combo photo shows Muslim Brotherhood candidate Mohammed Mursi (left) and former Prime Minister Ahmed Shafiq
Earlier on May 28, Egypt's election commission confirmed that Shafiq would face the Muslim Brotherhood's candidate, Mohammed Morsi, in a June 16-17 runoff for the presidency.
Morsi gained 24.3 percent of the vote in the first round of the poll held last week while Shafiq won 23.3 percent.
The commission put the official turnout in the presidential vote at 46 percent of the 50 million Egyptians who were eligible to vote.
It also denied allegations of fraud raised by two candidates who did not qualify for the run-off.
Based on reporting by Reuters, AFP, and AP