About 400 demonstrators, many of them waving Palestinian flags in red, white, green and black, chanted "murderers" as police looked on.
"Boycott Israel", "Peace" and "How many more have to die?" were among the signs on display wielded by demonstrators including university students, couples with their young children and pensioners.
The protest was organised by a coalition of over 40 peace groups and left-wing trade unions, including Spain's largest union Comisiones Obreras.
Fifteen Palestinians were killed Thursday when an Israeli shell slammed into a UN-run school in Gaza where hundreds of civilians had taken refuge, sending the Palestinian death toll from 17 days of fighting to 788.