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.
Sultan said Mursi had won with 24.77 percent of the votes, slightly ahead of Shafiq with 23.66 percent.
Nasserist candidate Hamdeen Sabbahi came third with 20.71 percent, ahead of moderate Islamist Abdel Moneim Abul Fotouh with 17.47 percent.
Former foreign minister Amr Mussa was fifth, trailing with 11.12 percent.
The commission put the official turnout in the vote -- the first since the 2011 uprising that ousted Mubarak -- at 46 percent 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 Mursi 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, however.
The Brotherhood has 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.
On Saturday, Mursi called a meeting of candidates that was ignored by both Sabbahi and Abul Fotouh.
He promised at a news conference after the meeting that his party would be prepared to include aspects of other parties' programmes in its platform, but fell short of reassuring critics who say the group wants to monopolise power.
"As president, I will be the president for all Egyptians. (My relationship) with the Brotherhood will be the same as all Egyptians," he said.
Shafiq also called on Saturday for broad support from former rivals, calling on his competitors by name to join him and promising there would be no return to the old regime.
Addressing the young people who spearheaded the 2011 revolt, he said: "Your revolution has been hijacked and I am committed to bringing (it) back."
The contest presents a difficult choice for activists who led the revolt. For them, choosing Shafiq would be to admit the revolution had failed, but a vote for Mursi could threaten the very freedoms they fought for.
The presidential poll has followed a tumultuous military-led transition from autocratic rule marked by political upheaval and bloodshed, but which also witnessed free parliamentary elections, which saw Egypt's two main Islamist parties clinch nearly three quarters of the 498 seats in the legislature.
The Supreme Council of the Armed Forces, in power since Mubarak's downfall, has pledged to restore Egypt to civilian rule by the end of June.