Greece election: Polls close in tight contest
Greece has held its fifth general election in six years, with a tight finish predicted between the left-wing Syriza party and conservative New Democracy.
Initial exit polls suggested a slight lead for Alexis Tsipras's Syriza.
The snap election was called after Syriza lost its parliamentary majority in August.
Mr Tsipras's popularity plummeted after he agreed a new bailout deal with European leaders.
The bailout involved austerity measures which Syriza had vowed to oppose.
A combined exit poll from five TV stations suggested Syriza would get between 30% and 34% of the vote, with New Democracy securing between 28.5% and 32.5% but the exit polls have in the past been unreliable.
Greece is mired in a deep financial crisis and whoever wins Sunday's election will have to oversee further tough economic reforms.
The BBC's Richard Galpin in Athens says whichever party wins is unlikely to get enough seats to form a government alone.
That could mean a period of political instability just as deadlines loom for the implementation of a series of key financial reforms, he adds.
Analysis: Richard Galpin, BBC News, Athens
Unlike last January's election, there is little excitement about Sunday's vote; campaigning has been lacklustre and the response of the electorate muted.
Syriza's high noon has passed, the party and its leader bruised by their experience in government.
Mr Tsipras's decision to abandon the anti-austerity stance which had propelled him into government, and instead sign a bailout deal with Greece's European creditors, fractured Syriza, with 25 MPs setting up their own parliamentary party.
Mr Tsipras said Greeks would elect "a fighting government" that will "move on with necessary reforms and break with the old regime", as he cast his ballot on Sunday morning in the Athens district of Kypseli.
The former Greek prime minister signed the bailout deal shortly after a referendum in which more than 60% of voters rejected the austerity measures creditors wanted to impose.
In interviews leading up to the election, Mr Tsipras said he had put his country above his party. He said that had he not agreed to the three-year bailout, Greece would probably have had to leave the eurozone.
He told Antenna TV on Friday he would "tug the rope" to try to win relief on Greece's huge national debt from EU creditors.
His main rival, New Democracy leader Vangelis Meimarakis, has dismissed Mr Tsipras's term in office as "an experiment that cost [the country] dearly".
"I fear that if Syriza is elected... the country will soon be led to elections again, and this would be disastrous," he said.
Greece's turbulent year
29 December: Greek parliament fails to elect president, leading to snap elections a month later
25 January 2015: Leftist Syriza party's Alexis Tsipras elected PM on an anti-austerity manifesto
24 February: Greece's European lenders agree to extend its second bailout by four months
June: Marathon talks take place to avoid Greece bankruptcy and possibly leaving the eurozone. PM Tsipras calls a referendum on a possible bailout agreement
5 July: Greek voters overwhelmingly reject terms of third EU bailout in referendum vote
14 August: Greece agrees €85bn (£60bn) bailout deal with its creditors - its third in five years - allowing tax hikes and new spending cuts. Mr Tsipras resigns a week later clearing the way for snap elections in September, as he seeks a new mandate
There is also a tight race for third place. Exit polls suggested the far-right Golden Dawn would take 6.5%-8% and the socialist Pasok party 5.5%-7%.
Analysts have said the migrant crisis on Greece's doorstep may boost support for Golden Dawn, which is strongly opposed to immigration.
The first projected results are expected within two hours.
Nearly 10m Greeks have registered to vote.