Unidentified assailants set fire to the headquarters of Egypt's runoff presidential candidate Ahmad Shafiq on Monday after officials said he would face a Muslim Brotherhood candidate in the second round.
An annex in Shafiq's headquarters in Cairo went up in flames hours after election officials announced that the former premier, a symbol of Hosni Mubarak's ousted regime, would square off against Brotherhood's Mohammed Mursi.
There were no immediate reports of injuries and firefighters said the blaze was quickly put under control.
"We were inside when they attacked us," one member of Shafiq's campaign staff said, without identifying himself. "They set fire to the garage that had general Shafiq's campaign literature."
Earlier around 1,000 protestors had gathered in Cairo's central Tahrir Square to protest Shafiq's presence on the runoff ballot.
Announcing the results, election commission chief Faruq Sultan said: "No candidate won an outright majority, so according to Article 40 of the presidential election law, there will be a run-off between Mohammed Mursi and Ahmed Shafiq."
The results exposed a deep rift within the nation, which now will have to choose between a conservative Islamist and a symbol of the hated Mubarak regime.
Egyptians went to the polls on May 23 and 24 in the country's first free presidential election made possible by the 2011 uprising led by pro-democracy activists.