A group of people carrying Spanish and fascist flags disrupted the celebrations at the Centre Cultural Blanquerna chanting "Catalonia is Spain".
They then set off pepper spray, leaving guests holding their mouths and noses and gasping for air.
Five people received other minor injuries during what Spain's Interior Minister Jorge Fernández Díaz described as "a barbarous attack".
Police later arrested five men and one woman aged from 23 to 56 for their part in the attack.
A senior leader of the ruling conservative Popular Party, Esteban Gonzalez Pons, denounced it as a "fascist attack".
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The vice-president of Catalonia's regional government, Joana Ortega, called it "totally intolerable".