The rally was smaller than the big demonstrations that took place just before Sunday's (local time) referendum but it underlined the deep resistance to fresh doses of austerity facing the government from the left.
Prime Minister Alexis Tsipras is seeking political support for the measures which he is promising European partners in exchange for a new 53.5 billion euro bailout deal to keep Greece in the euro.
He has secured backing from centrist and centre-right opposition parties but faces stiff resistance from many on the left of his own Syriza party.
"Syriza's platform was not about staying in the euro at any cost. This new deal they are cooking up has no prospect for growth, the austerity will humiliate us," said Yannis Kourtakis, 41, a supporter of Syriza's hard-line Red Network faction.
Demonstrators chanting slogans and waving banners with messages such as "Greece is not a colony" and "No, overturn the bailout" gathered near Syntagma Square in central Athens.
