Rival rallies ahead of Greece's referendum were held in central Athens late Friday, with a pro-government demonstration urging a "No" vote gathering 15,000 people and a nearby "Yes" crowd attracting 12,000, according to police.

The "No" rally in front of parliament was to be addressed later by Prime Minister Alexis Tsipras, who called the Sunday plebiscite on whether to accept bailout terms in a bid to strengthen his hand in negotiations with Greece's creditors.

The "Yes" demonstrators, who gathered in front of a stadium less 800 metres (yards) away, yelled out pro-European slogans, and warned that the referendum could see Greece crash out of the euro.