About 18,000 people have gathered in central Athens to support the newly elected government's push for a better deal on Greece's debt.
Participants at Sunday's rally outside the parliament in the Greek capital carried banners denouncing economic austerity and Greece's creditors.
"We want justice here and now... for all the suffering Greece has gone through the past five years," 58-year-old Theodora, who has been unemployed for the last three years, told the AFP news agency.
A protester wearing a mask of German Finance Minister Wolfgang Schaeuble and wielding a large plastic needle with "austerity" daubed on it jousted with a fake Greek Prime Minister Alexis Tsipras armed with a huge pair of red scissors.
"I'm not afraid of anything, whatever bad was going to happen has already happened," 62-year-old pensioner Vimitra told AFP, referring to five years of severe austerity in the debt-laden country.
About 8,000 people also thronged the streets in Thessaloniki, Greece's second biggest city in the north of the country, calling on Brussels to loosen the noose of austerity.
Similar rallies were also seen in several Greek cities and about 40 other solidarity gatherings were staged across Europe and in Australia, Brazil and the US.
In Paris, about 2,000 people walked through the city centre in a show of solidarity with Greece against what organisers called "the Goliath of finance".
Marchers unfurled flags from the far-left Syriza party of their new prime minister and chanted: "In Greece, in France, resistance against austerity and finance."
In Lisbon, about 300 people took to the streets with banners reading "Greece, Spain, Portugal, our battle is international."
"We want to express our solidarity with the Greek people to defend their right to decide their own future," said Paulo Coimbra, a 46-year old economist and one of the organisers of the march.
"We contest austerity in Portugal as well, it's time to say 'enough' to the grip financial power has on political power," he added.
The Greek government enthusiastically welcomed these rallies while insisting that they were spontaneous affairs, organised through social media.
The pro-government rallies came a day before a gathering of eurozone finance ministers who will consider Greece's proposal for short-term "bridge financing" without the onerous terms previously imposed on the country, until a longer-term solution to Greece's crushing debt is found.
So-called technical level talks with creditor representatives ended on Saturday, Greek officials said.
Greece is hoping to renegotiate its 240bn euro ($274bn) bailout, but its overtures to its international creditors have so far fallen on deaf ears, with Germany unwilling to reverse its policy on austerity.
Athens risks being forced out of the eurozone if a deal is not found by the end of the month.