Around 16,000, mainly Communist party-affiliated, demonstrators marched in Athens while another 4,500 protested in Thessaloniki, local police said.
Around midday, about 10,000 striking workers, pensioners and students marched peacefully to parliament where lawmakers have been debating the 2018 budget. A vote on the draft is scheduled for December 22.
Some of the protesters held banners in support of Palestine and against the US decision to name occupied Jerusalem as the capital of Israel.
The two unions have staged repeated strikes since the first bailout in 2010, but participation has been low in recent years due to inertia and austerity fatigue after three rescue packages and five snap elections in eight years.
Sources: Reuters, AFP