If no such clear winner emerges on the first ballot, however, the two leading vote-getters will face a run-off election.
CNN: ASIANOW - TIME Asia | Indonesia: Taking It Right Down to the Wire
Mr Mousavi was hoping to prevent Mr Ahmadinejad winning more than 50% of the vote, in order to force a run-off election.
In the capital, Kinshasa, where three days of fighting greeted the announcement, in August, of a run-off election, people are visibly on edge.
On November 26, a run-off election will take place in Ecuador that will determine who the next President of Ecuador is going to be.
Two Democratic candidates will face one another in a run-off election to succeed Antonio Villaraigosa as mayor of Los Angeles, after neither won a majority in Tuesday's vote.
BBC: Run-off election for Los Angeles mayor after split poll
The latest polls suggest no candidate will win more than 50% of the vote, in which case the top two will face a run-off election on 4 November.
But by falling short of an outright win, Mr Perez Molina seems set to face Manuel Baldizon, who garnered some 23%, in a run-off election on 6 November.
BBC: Guatemala set for run-off vote amid concerns over crime
Former leftist guerrilla Jose Mujica took up office on 1 March 2010, after having won a run-off election against conservative former President Luis Lacalle in November 2009 with about 53% of the vote.
This week, a conversation with Senator Joe Lieberman, who's defending his seat against opponent Ned Lamont in Connecticut's Democratic Primary next Tuesday, a run-off election in Congress Woman's Cynthia McKinney's District also coming up.
In lieu of official results, ZESN released exit polling data that showed Tsvangirai leading with more than 49 percent of the vote -- short of the 50 percent plus one vote needed to avoid a run-off election.
The opposition, which says it is being targeted in attacks by government forces, claims that the weapons would have been used to intimidate voters in the event of a run-off election as Mugabe attempts to shore up his support.
Mr Mwanawasa has been one of the strongest African critics of Zimbabwe, where President Robert Mugabe won a disputed run-off election last month.
The UN Security Council on Monday unanimously agreed to condemn the violence in Zimbabwe and said a free and fair run-off election would be "impossible".
In the run-up to the Haiti elections yesterday, the OAS played a pivotal role in ensuring a free, fair run-off election involving the candidates who actually got the most number of votes in the first round of the election.
Mr Karzai was declared Afghan president after a second round election run-off was abandoned when his sole remaining challenger pulled out, saying the vote could not be free and fair.
In the 1996 Senate race, fear that he would make it to the run-off in a presidential-election year prompted state Republican leaders to step in and anoint their own candidate.
Only seven appeared to have won re-election without a run-off.
Michel Martelly, who first made his name on the Haitian music scene, was inaugurated as president in May 2011 after coming out of nowhere to win 68% of votes in the run-off of a hotly-contested presidential election in March.
By acquiescing in a run-off, however, he would risk giving Mr Milosevic a chance to steal the election for good.
Former Czech Prime Minister Milos Zeman is set to face Foreign Minister Karel Schwarzenberg in a run-off in the Czech Republic's presidential election.
The Taliban called for a boycott Saturday of Afghanistan's upcoming presidential run-off election and repeated threats to disrupt the vote.
On current polls, the election is heading for a run-off on October 25th between Francisco Rossi and Paulo Maluf, two old-fashioned populists.
At a comparable point ahead of the 2002 presidential election, in which he won 17% and a place in the run-off, he was scoring only 6%, according to Ipsos, a polling agency.
The run-off will be a very different affair from last Sunday's election, one that will be much more difficult for Mr Menem to fight.
On Saturday the election commission said Mr Kenyatta had narrowly avoided a run-off by winning 50.07% of votes in a credible and transparent poll.
BBC: Kenya election: Uhuru Kenyatta hails 'democracy triumph'
Mr Abdelaziz won nearly 53% of the votes cast in the first round of an election on July 18th, forgoing the need for a run-off.
He now wants to introduce a presidential run-off and reduce the time before being eligible to stand for re-election, which would suit his own political interests.
The Democrats are still sore from their experience there in the presidential election two years ago: after a six-week run-off involving bitter legal wrangles, the result in Florida tipped the balance in favour of Mr Bush.
The defeated Green Party candidate in Brazil's presidential election has said she will remain neutral before a second round run-off vote on 31 October.
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