Sheikh Ali Salman, secretary general of the al-Wefaq national Islamic movement, was arrested last December after calling for political reform, where the Sunni Al-Khalifa monarchy rules over a Shia majority.

Bahrain has faced unrest since 2011 demanding a constitutional monarchy and more representative government. At least 89 people have been killed in clashes with security forces. Hundreds have been arrested and put on trial.

Amnesty International said: “Today’s verdict is shocking. It is yet another clear example of Bahrain’s flagrant disregard for its international obligations. Sheikh Ali Salman has been sentenced solely for peacefully expressing his opinion.”

Ali al-Aswad, a former Wefaq MP, said: “The message from Bahrain is crystal clear: for at least the next four years there will be no dialogue in Bahrain. Locking away the leader of the main opposition party, a figure who has done nothing but promote peacefulness, means the door to reform is firmly locked.

Human Rights First said: “Sending the leader of the main opposition group to jail for peaceful protests only encourages those pushing for violent change.”

Al-Wefaq boycotted Bahrain’s elections last November on the grounds that promised reforms had not taken place. Unlike other Bahraini opposition groups that have demanded the overthrow of the Al Khalifa, al-Wefaq wants a constitutional monarchy.

Brian Dooley, of HRF, said: “Silencing peaceful dissent is a script we’ve seen before, from Egypt to Guatemala to Zimbabwe. It never ends well.”