Greek police have fired tear gas at anti-fascist protesters who hurled petrol bombs and stones near the Athens headquarters of the far-right Golden Dawn party.
The trouble followed the killing of an anti-racism rapper by a Golden Dawn supporter.
Reuters reports thousands of Greeks marched towards the party's offices in the biggest show of public anger over the stabbing.
Golden Dawn is Greece's third most popular party and the most visible symptom of discontent over state corruption and a deep economic crisis that has fuelled hostility to immigrants.
But polls since the killing of rapper Pavlos Fissas indicate the party has lost about a third of its public support.
Wednesday's rallies by university students, labour unions and leftists in Athens and the second biggest city Thessaloniki have overshadowed a 48-hour public sector strike over layoffs demanded by Greece's international lenders.
More than 10,000 people, some holding banners reading "Never Again Fascism" and singing anti-fascism anthems, marched outside parliament in the central Syntagma Square, where hip-hop groups held a memorial concert earlier on Wednesday.
The protest column then headed for Golden Dawn's headquarters five kilometres from central Athens.