Protesters are carrying banners denouncing economic austerity and Greece's creditors.
Similar rallies are taking place in several Greek cities and about forty other solidarity gatherings are planned across Europe and in Australia, Brazil and the US.
The Greek government has enthusiastically welcomed these rallies while insisting that they are spontaneous affairs, organized through social media.
On Monday, a gathering of Eurozone finance ministers 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 Saturday, Greek officials say.