Greeks are going to the polls to elect a new parliament, with the centre-right opposition mounting a strong challenge to the leftist government.
The New Democracy party of Kyriakos Mitsotakis is hoping to end more than four years of rule by Prime Minister Alexis Tsipras’s Syriza party.
Mr Tsipras called snap elections soon after suffering an electoral defeat in May’s European elections.
Polling stations opened at 07:00 local time (04:00 GMT).
It is Greece’s sixth election since the global financial crisis in 2008.
The crisis triggered a succession of financial bailouts, with the Greek economy shrinking by 28% between 2008 and 2016, and increasing unemployment has thrown many Greeks into poverty.
Greece exited the bailout programme in August of last year and growth has returned.
But with temperatures hitting 35C in much of the country, many politicians are concerned on the impact the weather may have on turnout, as voters stay cool at home – or head to the beach.
What are the rival parties offering?
Mr Mitsotakis is promising lower taxes, greater privatisation of public services and plans to renegotiate a deal with Greece’s creditors that would allow more money to be reinvested in the country.
Mr Tsipras, who came to power in 2015, has promised more investment and recently boosted pensions. His own investment policies would also have to be renegotiated with creditors as the country remains under eurozone supervision.
Each of the country’s numerous parties needs to gain at least 3% of the vote to get into the parliament and as many as seven of them could win seats.
The winning party gets a 50-seat bonus and needs 151 seats in the 300-seat parliament to have a majority.
At the European elections, New Democracy won 33.11% of the vote against 23.78% for Syriza.
The highest percentage of 18-to-24 year olds (30.5%) at that election backed New Democracy.