About 3,000 people took part in the protest outside the parliament building, disrupting public transportation for about five hours while police diverted traffic away from Syntagma Square. They planned to stay until lawmakers voted on the proposed hiring system.
Greece's left-wing government has promised to hire 15,000 full-time teachers over the next three years, starting this fall. Unions say the hiring system is unfairly weighted against teachers who have worked for years on short-time contracts. They also want more teachers to be hired.
The umbrella civil servants' union, Adedy, called a 24-hour public sector strike for Thursday to back the teachers' demands, although the walkout mainly affected schools. Classes were canceled in most public schools in Athens and many were elsewhere in Greece.
A few dozen protesters briefly clashed with police officers blocking their access to parliament, but no injuries or arrests were reported. Teachers previously held two protest marches in Athens over the hiring plan. Both resulted in clashes between some demonstrators and riot police.