Demonstrators furious that Hosni Mubarak's last prime minister made it into the run-off for the country's presidential election set ablaze his campaign headquarters on Monday, underscoring the divisive outcome of the nation's historic vote. The campaign offices of Ahmed Shafiq, viewed as a symbol of Mubarak's rule, were set on fire after a group of protesters broke into and vandalized the premises, 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 first-round result - a run-off between Shafiq and the Muslim Brotherhood's Mohamed Mursi, two of the most controversial figures in the field.

Troubled flared in Cairo's Tahrir Square when activists said unknown assailants attacked one such protest. Rocks were thrown in scenes reminiscent of other spasms of violence in a messy transition from military rule that is due to end with the election of the president.

The April 6 movement, one of the group's that spearheaded the 2011 revolt against Mubarak, said on its Facebook page that the Tahrir protest had been attacked by unknown "thugs".

Analysts had predicted that a Shafiq-Mursi run-off could trigger trouble, leading to 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.

The result is deeply disappointing to the activist movement that took to the streets on January 25, 2011, inciting the protests that toppled Mubarak. They had seen other candidates as more representative of their hopes for change.

One of those candidates, Khaled Ali, joined a protest in Tahrir Square, where the numbers grew into the night.

"Revolutionaries! Free! We will complete the march!" chanted some 2,000 demonstrators as they made their way through the centre of Cairo, a short distance from Tahrir Square, the cradle of the uprising that toppled Mubarak.

Though both Mursi and Shafiq have sizeable constituencies, the result has left the many Egyptians who voted for neither with a wrenching choice between a symbol of the past autocracy and an Islamist group that arouses deep suspicions for some.

Mursi topped the poll with 24.3 percent of the vote, followed by Shafiq with 23.3 percent. Turnout was 46 percent, according to official results released on Monday.

About half of the first-round votes went to candidates somewhere in the middle ground - from leftist Hamdeen Sabahy, third-placed with 20.4 percent, to moderate Islamist Abdel Moneim Abol Fotouh, with 17.2 percent, and former Arab League secretary-general Amr Moussa, with 10.9 percent.

"Neither Brotherhood or feloul," said Mahmoud Momen, a 19-year old student, invoking the word used in Egyptian political slang to refer to politicians who served in the Mubarak administration. He had voted for Abol Fotouh.

"We want someone who represents the square," added Momen, holding aloft a picture of Shafiq with black X daubed over his face as he took part in the Cairo march.

Another protester, a 19-year old student who identified himself as Omar, said the vote had been rigged, triggering an argument with a bystander who disputed the claim.

Similar protests erupted in Alexandria on Egypt's northern Mediterranean coast and Port Said, Ismailia and Suez, cities along the Suez Canal east of Cairo.

In Alexandria, some 2,000 protesters marched through the city, tearing up Shafiq and Mursi election posters they encountered along their way.

A MESSY TRANSITION

Abol Fotouh, Sabahy and Moussa filed complaints about the voting, all of which were rejected by the six judges forming the electoral committee.

The disputes add rancor to an already messy and often bloody transition to democracy since generals took over from Mubarak when a street revolt forced him out on February 11, 2011.

"I reject these results and do not recognize them," said Abol Fotouh, a former Brotherhood member, alleging that votes had been bought and representatives of candidates had been denied access to polling stations during the count.

Moussa said earlier that "question marks" hung over the vote. "There were violations, but this should not change our minds on democracy and the necessity of choosing our president."

The Muslim Brotherhood sought to muster a coalition to help Mursi against Shafiq, who calls Mubarak a role model.

The close contest has set both contenders scrambling for support, particularly the Muslim Brotherhood, which is trying to draw losing candidates and other political forces into a broad front to prevent a "counter-revolutionary" Shafiq victory.

The ultra-orthodox Salafi Islamist party Al-Nour has said it will now back Mursi, after siding with Abol Fotouh in the first round. The party has the second biggest bloc in parliament after the Brotherhood's Freedom and Justice Party.

Beyond the Islamist movement, it might prove harder for the group to find allies. Secular-minded parties have grown suspicious of the Brotherhood, accusing it of being power hungry and putting a quest for power over principle - charges it denies.

A Brotherhood source, who asked not to be named, said the Islamist group's FJP party had prepared a menu of options to tempt rival groups and politicians to its side.

These include creating a five-member advisory council to advise the president; assigning the posts of prime minister or vice-president to Abol Fotouh and Sabahy; distributing cabinet posts to other parties and offering compromises on planned laws and on an assembly tasked with drafting a new constitution.

Shafiq is also seeking wider backing, even posing as a protector of the revolt that toppled Mubarak.

Shafiq's supporters see him as the man to impose security and crack down on protests viewed as damaging to the economy.

Mursi appeals to Egyptians who believe the Islamists are best qualified to reform a corrupt state.