Demonstrators furious that Hosni Mubarak's last prime minister made it into the run-off for Egypt's presidential election set ablaze his campaign headquarters on Monday, May 28 witnesses said, underscoring the divisive outcome of the country's historic vote, Reuters reported.

Former air force commander Ahmed Shafiq, who has described Mubarak as a role model, will face the Muslim Brotherhood's Mohamed Mursi in the run-off. It is a contest between the two most polarizing and controversial figures in the race.

A group of protesters broke into and vandalized Shafiq's office in the residential district of Dokki before setting it ablaze, the state news agency reported. An official in the fire service confirmed the blaze had been extinguished without causing any casualties.

Several thousand protesters took to the streets across Egypt to demonstrate against the result of the election's first round, which was officially announced on Monday.

Trouble flared in Cairo's Tahrir Square when activists said unknown assailants attacked one such protest. Rocks flew in scenes reminiscent of other spasms of violence during a messy transition from military rule that is due to end when the military hands power to the new president on July 1.

Local media reported the protest had been attacked by unknown "thugs", though the account could not be independently confirmed.

Many analysts had predicted that a Shafiq-Mursi run-off could trigger trouble. The vote marks a ballot box struggle between a symbol of the military-based autocracy of the last six decades and one of the Islamist movements it had oppressed.