Counting is under way in Australia in a general election seen as the tightest in the country for decades.With over half of votes counted, projections by Australia's ABC put PM Julia Gillard's Labor party slightly ahead of the opposition coalition.
But the results also suggested a swing against Labor in parts of the country, and talk of a hung parliament.
The vote takes place two months after Ms Gillard ousted Kevin Rudd in a controversial leadership challenge.
Unofficial figures by ABC, based on 52.7% of votes counted, Ms Gillard has more seats but only 38.1% of votes, compared with 43.7% garnered by conservative coalition leader Tony Abbott.
Analysts say Australia could be heading for a hung parliament, a situation which would arise if neither contender gains 76 or more seats. The country's last hung parliament was in 1940.
As results come in, party officials have said it is becoming "more and more likely".
Early results indicated Labor suffered heavy swings against it in key states of Queensland and New South Wales.
Fourteen million registered voters began casting their ballots at 0800 (2200 GMT). Voting is compulsory in Australia.
While first early exit polls put Ms Gillard ahead of Mr Abbott by a slim margin, analysts warned that the swing in key marginal seats would determine the final vote.
A Channel Nine exit poll suggested an expected Labor win of 52% compared to 48% for Mr Abbott's coalition.
A second exit poll by Sky News indicated Ms Gillard would garner 51% of the votes to Mr Abbott's 49%.
"It all depends on the uniformity of the vote across the country," David Briggs of the Galaxy polling company told AFP news agency.
Speaking on Australia's ABC television channel, Foreign Minister Stephen Smith said the election would be decided by "30 or more marginal seats throughout the country".
Earlier, Mr Abbott declared that it was "a day when we can vote out a bad government".
"It's a day when we can vote in favour of a competent stable government which respects the tax payer's dollar," he said while casting his vote in Sydney.
Ms Gillard, who voted near her modest house in suburban Melbourne, told reporters that it was a tight race.
"This is a tough, tight, close contest, but I'm exercising my own vote," she said.
Mr Abbott - who leads the Liberal Party - worked through the final night of the campaign.
Correspondents say he has tried to exploit the Labor party's divisions after the departure of Mr Rudd, trying to portray his coalition as a stable answer to a government beset by in-fighting.
In his campaign he has pledged to tighten immigration and has hit out at government spending. He has also toned down his well-known climate change scepticism.
Ms Gillard, a former lawyer who called a snap election shortly after coming to office, is hoping to be rewarded for the government's handling of the economy, which weathered the global recession remarkably well.
During campaigning, both candidates made several visits to marginal constituencies upon which - given the slim margin between them - the election could hang. Most of these are in the states of Queensland and New South Wales.
All of the seats in the lower house are up for grabs, plus half of the seats in the Senate - where opinion polls show the Greens could end up holding the balance of power.
That Labor is locked into such a tight election race represents a turnaround in its fortunes since the start of the year.
Missteps by Kevin Rudd on climate change and a controversial mining tax caused his support - previously high - to fall sharply.
Ms Gillard was Mr Rudd's deputy, and when she challenged him for the leadership in June he surrendered without a fight after realising his support among government colleagues had collapsed.
But Ms Gillard's move triggered a backlash, with support for her falling sharply in the two months since she has been in office.