Some 4,500 protesters gathered in Athens central Omonoia Square on 24 July and then marched toward Syntagma Square, disrupting traffic, the police said in a statement.
"Suddenly, some of those gathered began to throw pieces of marble, stones, flares and Molotov cocktails at police officers ... The police forces began to disperse the crowd with water cannons and limited use of tear gas," the statement issued on Saturday read.
A total of 25 people were detained in the incident, the police added.
Earlier in July, the Greek parliament passed a bill that makes COVID-19 vaccination mandatory for employees in some specific sectors such as healthcare, sparking a public outcry. Saturday's demonstrations were the third such event in recent days.