Left-wing political figures including the leaders of the Left Party, a mix of former East German communists and defectors from the Social Democrats (SPD), took part in the annual ceremony at a cemetery in Berlin's Friedrichsfelde suburb.
Police said 15,000 turned out to honour Karl Liebknecht and Rosa Luxemburg, who were kidnapped and killed on January 15, 1919.
Earlier, as many as 10,000 demonstrators marched to protest "against capitalism, war, anti-social policies and right-wing extremism," with representatives of Germany's Palestinian community in attendance, according to police.
Numbers have declined in recent years, but after the fall of the Berlin Wall in 1989 up to 100,000 people turned out to mark the anniversaries of the assassination of Liebknecht and the Polish-born Luxemburg, who is considered a Marxist martyr.
In 1918, they had transformed their Spartacus League into the German Communist Party and tried to proclaim a German Soviet Republic.
When the outfit organised an uprising in Berlin the following January, it was brutally supressed by nationalist militia and the remnants of the German imperial army.
Liebknecht and Luxemburg were seized, beaten and shot in the head. Luxemburg's body was dumped in a canal and only found months later.