Michelle Bachelet has taken a lead in the race to become president of Chile and will face a strong right-wing challenge in a 15 January run-off.
Opinion polls have suggested Ms Bachelet would triumph in a run-off
The candidate of the centre-left governing coalition - hoping to become Chile's first woman president - won nearly 46% of the vote, officials said.
Sebastian Pinera will be her rival in the run-off, after third-placed Joaquin Lavin threw his support behind him.
The government coalition also posted solid gains in congress.
It is the first time the Concertacion bloc has won a majority in the Senate since it came to power in 1990 following the end of General Augusto Pinochet's military rule.
Michelle Bachelet 45.9%
Sebastian Pinera 25.4%
Joaquin Lavin 23.2%
Tomas Hirsch 5.4%
With 99.3% of ballots counted
The two presidential candidates from the right fought a close contest on Sunday. Millionaire businessman Pinera had 25.4% of the vote, and former Santiago major Lavin 23.2%.
After admitting defeat, Mr Lavin announced on television he was going to express his "total support" to Mr Pinera.
The two men's combined vote suggests the run-off could be tight.
Commentators say the 5.4% cast for fourth-placed Humanist candidate Tomas Hirsch is likely to go to Ms Bachelet.
"The game is wide open," political analyst Eugenio Tironi told the AFP news agency.
"We'll see a race against the clock to recover the votes of Lavin and Hirsch."
'Women work harder'
Michelle Bachelet thanked her followers and promised she would win in January.
"I would have liked to have won in the first round," she admitted.
"I take this as a reason to work harder, to push ourselves even harder, because after all, women are used to working twice as hard," she added to loud applause.
Voters were enthusiastic
Mr Pinera, a former senator, promised: "There will be a second round and it will be Michelle Bachelet on one side, leading the somewhat worn-out ruling bloc, and me on the other side heading a new, younger, stronger coalition."
If Ms Bachelet wins in January, she would find a supportive Congress after the Concertacion bloc won a majority in the Senate (55.7%) and expanded its current majority in the Chamber of Deputies to 51.7%.
The BBC's Daniel Schweimler in Santiago says that after their long years of military rule, Chileans take their democracy seriously and turnout appears to have been high.
Voting is compulsory among Chile's more than eight million registered voters.
Shopping centres and cinemas remained closed and there were no sporting, cultural or public events. The sale of alcohol was forbidden.
The campaign has been dominated more by personalities than by issues, our correspondent says.
Ms Bachelet, a former defence minister and a single mother, is the candidate for the Concertacion coalition, which has been in power since the end of the 1973-1990 Gen Pinochet rule.
Chile's presidential contest has been a fight between personalities
Her family history makes her a popular figure among many Chileans. She is the daughter of an air force general tortured to death by Pinochet's secret police.
Two years after his death, she and her mother were arrested and tortured.
Opinion polls have suggested the former defence minister would win a run-off against any other candidate.
If so, she is expected to follow many of the policies of President Ricardo Lagos - free-market economics mixed with leftist social programmes.
The other candidates have focused on Chile's rising crime, high rates of unemployment and a wide divide between rich and poor.
Our correspondent says despite these issues, Chile is still a beacon of economic and political stability in an otherwise troubled region.