Police in Athens scuffled with protesters opposed to shops opening on Sunday, a week before Orthodox Easter.
The incident occurred on the capital's main shopping street.
Several dozen protesters had unfurled banners in front of businesses, urging people to refrain from going inside.
"Hands off the Sunday holiday, resist Middle Age practices in labour," read one banner.
Unions say that allowing shops to open on Sunday will mainly benefit large chains, to the detriment of small family-owned businesses already suffering from a six-year recession.
Greece recently passed legislation to permit allow retail shops to operate on Sunday, conditional on the agreement of local town councils.