Greek Prime Minister Alexis Tsipras says he's 'optimistic' about the outcome of the country's bailout referendum ashe casts his ballot at a polling station in the Kypseli neighbourhood of central Athens, dpa reported.

'Today is a day of celebration because democracy is a celebration,' he says, adding that the decision is in the hands of the people.

'Many may ignore the will of a government but not the will of the people,' he says.

'I am confident that tomorrow we will open a way for all the people of Europe,' he says.