Athens (AFP) - Some 17,000 people took to the streets of Athens and Thessaloniki on Monday to say 'No' in an upcoming referendum on the latest bailout proposals, accusing Greece's international creditors of blackmail.

"Our lives do not belong to the creditors!" read banners held aloft by demonstrators, many of whom support Prime Minister Alexis Tsipras and said they would heed his call to vote against the latest deal in Sunday's referendum -- despite risks it could send the country crashing out of the eurozone.

"The people of Greece have made many sacrifices. What interests me is not the euro but guaranteeing a dignified way of life for the next generations," 50-year-old Vanguelis Tseres, who has been unemployed since the start of the debt crisis in 2010, told AFP in Syntagma square in the capital.

European leaders had earlier Monday pleaded with Greek voters to back the hotly disputed bailout proposals or face leaving the euro.

European Commission chief Jean-Claude Juncker warned Greeks they "shouldn't choose suicide just because you are afraid of death," urging them to vote 'Yes' as a 'No' vote would be a no to Europe.

But Loula Bakrina, a petite blonde, said she "will say no because I want my children to be free."

Police estimated some 13,000 people were gathered in front of parliament in Athens, with another 4,000 in Thessaloniki, the country's second largest city.

A counter-demonstration in favour of the bailout deal was due to be held on Tuesday -- the same day the country's current bailout deal expires and it is expected to default on a key debt payment.

"Today, those who stand tall are here. Tomorrow it will be those on all fours," demonstrator Trintafilos said, mocking people expected to gather in Syntagma in a rally against the government's handling of the crisis.

Tsipras shocked EU leaders by announcing his intention to put the latest bailout offer to a popular vote, effectively thrusting a stick in the negotiation wheels despite a June 30 deadline to repay money to the International Monetary Fund (IMF).

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The radical left, which came to power in January promising a new bailout deal, insists Greece has suffered enough austerity, with Tsipras accusing international creditors of wanting to "humiliate" the country.

Greece has already endured five years of recession, while unemployment has more than doubled from since 2009 to 25.6 percent this year and pensions and benefits have been roughly halved.

"All these years we've being saying yes, it's about time we say something different, just to see what happens," said artist Andonia Klogirou, demonstrating in Athens.

Nearby, teacher Pinalope Sinodinou said she would vote no at the referendum "so we can have our dignity back. We want to be able to have our own opinions, not ones that are forced upon us."