A mob has set fire late to the campaign headquarters of one of the two Egyptian presidential politicians facing each other in a run-off that will decide a new leader after last year's popular uprising, the first sign of unrest after the voting yielded divisive candidates.

The late Monday attack on Ahmed Shafiq's office came just hours after the country's election commission announced that he would face the Muslim Brotherhood's candidate, Mohammed Morsi, in a June 16-17 run-off.

The second round pitting Shafiq, who was ousted president Hosni Mubarak's last prime minister, against Morsi, backed by the country's most powerful Islamist movement, is a nightmare scenario for the thousands of Egyptians who took to the streets last year to demand regime change, freedom and social equality.

Many of the so-called revolutionaries say they want neither a return to the old regime nor religious rule.

"The choice can't be between a religious state and an autocratic state. Then we have done nothing," said Ahmed Bassiouni, 35, who was sitting in Cairo's downtown Tahrir Square in the midst of a growing protest.

In an upscale neighbourhood of Cairo, mobs of young men used bricks to smash the windows of Shafiq's headquarters, tossing out campaign signs and tearing up his posters. Then they set fire to the building. There were no reports of injuries. Police arrested eight people.

His campaign blamed supporters of leftist candidate Hamdeen Sabahi, who came in third in the race, and backers of another losing candidate, Khaled Ali, who was protesting the election results Monday evening in Tahrir Square, the centre of last year's uprising.

Shafiq, also a former air force commander, was forced out of office as prime minister by protesters shortly after Mubarak's fall. He has since presented himself as a figure who can restore calm to a country wracked by 15 months of sometimes violent protests and deterioration in internal security. He has expressed a zero-tolerance attitude toward protests, reflecting his background in the military and in the former regime, which put down protests with brutal force and jailed opponents.

Shortly after the protesters ransacked the campaign office, fire trucks and police arrived as several hundred of Shafiq's supporters gathered outside the building, carrying his picture and chanting slogans against the Muslim Brotherhood, which controls the parliament and is now seeking the presidency.

"The Brotherhood are enemies of God!," chanted the crowd.

The Morsi-Shafiq run-off is a polarizing contest. It mirrors the conflict between Mubarak, himself a career air force officer like Shafiq, and the Islamists he jailed and tortured throughout his years in power. But it sidelines the mostly young, secular activists who led the popular uprising last year.

Official results released

The commission reported Monday that Morsi won close to 5.8 million votes, or almost 25 per cent, while Shafiq received 5.5 million votes, or nearly 24 per cent. Sabahi, a socialist, finished third with 4.8 million votes, or about 21 per cent. Fourth place went to moderate Islamist Abdel-Moneim Abolfotoh. Turnout was about 50 per cent.

In Tahrir Square, several thousands protesters chanted slogans against the military rulers who took over after Mubarak's ouster. Protesters have clashed frequently with the military in street protests that have killed more than 100 people, charging that the military is perpetuating the repressive practices of the Mubarak regime and bungling the transition to a new, elected government.

Protesters also chanted slogans against both Morsi and Shafiq, saying they will not allow Egypt to be ruled by one party again nor allow the former regime to regain power.

"Freedom! Freedom!" the crowds chanted, fists pumping in the air.

Some were demanding that a law approved by parliament banning former high-level regime officials from running in the election be implemented. That could apply to Shafiq. Egypt's constitutional court is set to look at the law just four days before the run-off.

Allegations of fraud rejected

Others charge that last week's election, with 13 candidates, was rigged, though observers said the vote was generally free. In the Mediterranean port city of Alexandria, where Sabahi, a favourite among many revolutionaries, won the most votes, protesters tore down and burned large Shafiq and Morsi posters and protested against military rule.

In the Nile Delta provinces of Dakahliya and Mansoura, protesters took to the streets in similar protests. Security officials said protesters in Mansoura tried to attack the campaign offices of Morsi and Shafiq, but supporters of both candidates stopped the crowd.

The protests come just one day after Sabahi and Abolfotoh, whose supporters backed the popular uprising, filed appeals to the election commission to delay announcing the first round results until allegations of voter fraud could be investigated. Their appeals were rejected Monday.

Speaking to reporters Monday, Abolfotoh said "violations threatened the integrity of the election."

"It is impossible under any circumstances for me to say with a national conscience that these elections were clean," Abolfotoh said.