Unidentified assailants have set fire to the headquarters of Egypt's runoff presidential candidate Ahmad Shafiq and thousands of protesters have returned to Cairo's Tahrir Square to rally against alleged injustice in the election process.
An annex in Shafiq's headquarters in Cairo went up in flames late on Monday, hours after election officials announced that the former prime minister, a symbol of Hosni Mubarak's rule, would square off against Brotherhood's Mohammed Morsi.
There were no immediate reports of injuries and firefighters said the blaze was quickly put under control. Police arrested eight people in connection with the attack.
"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 2,000 protesters had gathered in Cairo's central Tahrir Square to protest Shafiq's presence on the run-off ballot.
Posters torn up
Al Jazeera's Jamal Elshayyal, reporting from Cairo, said several hundred protesters marched during the day in the city of Alexandria, tearing up posters of Shafiq. Protests were also reported from the Nile Delta provinces of Dakahliya and Mansoura.
In Cairo's Tahrir Square, two presidential candidates, including Mohammed Morsi, the Muslim Brotherhood candidate, joined the protesters.
"If [the elections] were unpredictable to start with, they are even more chaotic now," our correspondent said.
"These protests have brought to light that the Egyptian people are polarised."
Protesters 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.
"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 Tahrir Square in the midst of a growing protest.
Announcing the results earlier on Monday, Faruq Sultan, election commission chief, said: "No candidate won an outright majority, so according to Article 40 of the presidential election law, there will be a runoff between Mohammed Morsi and Ahmed Shafiq."
The results exposed a deep rift within the nation, which now will have to choose between a Islamic conservative 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.
Sultan said Morsi had won with 24.7 per cent of the votes, slightly ahead of Shafiq with 23.6 per cent.
Nasserist candidate Hamdeen Sabbahi came third with 20.7 per cent, ahead of moderate Islamist Abdel Moneim Abul Fotouh with 17.4 per cent.
Former foreign minister Amr Mussa was fifth, trailing with 11.1 per cent.
The commission put the official turnout in the vote, the first since the 2011 uprising that ousted Mubarak, at 46 per cent of the 50 million Egyptians who were eligible to cast a ballot in the historic election.
Sultan said the commission had rejected seven appeals filed by candidates on May 26 and 27, citing electoral irregularities that "did not affect the outcome of the vote".
Both Morsi and Shafiq, who represent polar opposites in the country's fragmented politics after last year's uprising, are now trying to court the support of the losing candidates and their voters.
The Brotherhood, which alienated many other political parties after its domination of parliamentary elections last winter, has warned that the nation would be in danger if Shafiq wins and has pledged to become more inclusive.
Two of the losing candidates, Mussa and Abul Fotouh, declined to endorse either of the frontrunners.
The Brotherhood has, however, gained the support of the ultra-conservative Salafist Al-Nur party, which had supported Abul Fotouh in the first round.
But a pending legal case could have serious implications for Shafiq's bid for the presidency.
Egypt's Supreme Constitutional Court is expected to rule on June 11 in a key case examining the constitutionality of a law barring senior Mubarak-era officials from running for office.