Rights group Amnesty International has renewed its call for the release of the jailed president of the Bahrain Teachers Association, Mahdi Abu Dheeb.
Mr Abu Dheeb was convicted by a military court of plotting to overthrow the government during unrest that swept Bahrain in 2011.
His original 10-year sentence was subsequently reduced to five on appeal.
Amnesty has described Mr Abu Dheeb as a "prisoner of conscience".
Both Mr Abu Dheeb and his vice-president Jalila al-Salman allege they were tortured in detention after calling for a strike by teachers in March 2011 in support of pro-democracy activists who had taken over a prominent landmark, Pearl Roundabout, in the capital, Manama.
The Bahrain Teachers Association was dissolved by the government after its leaders were arrested.
Ms Salman was originally sentenced to three years in jail but that was reduced to six months on appeal.
However in March this year she was sacked from her teaching job after criticising Bahrain's human rights record at a conference in Washington DC.
In a statement timed to coincide with 1 May, International Workers' Day, Amnesty said: "All that they did was call for a strike in their role as trade union leaders. Mahdi and Jalila were punished for doing their job. This May Day stand with workers across the world and demand Mahdi's release."
Mr Abu Dheeb's daughter, Maryam, in a recorded message accompanying the statement, said: "Silence is a crime."
She urged people to speak up and "take a step and show you care about what is going on in my country".
Teachers' organisations around the world, as well as human rights campaigners, have called for Mr Abu Dheeb's release.
The Bahraini authorities did not respond to a request from the BBC to comment on the Amnesty International statement.