Protests held as part of the strike in the capital, Athens, on Tuesday turned violent after hundreds of young protesters, many wearing gas-masks and scarves covering their faces, hurled rocks and pieces of broken masonry at riot police. Some reports indicated that protesters had also thrown homemade fire bombs.
The police responded by firing tear gas at the protesters, detaining 18 of them and laying formal arrest charges against three.
Twenty-one police officers were injured in the clashes, officials said. They also said that one person had been stabbed during fights between rival groups of protesters, with three others left injured.