A prominent Bahraini activist has been jailed for three years for instigating and participating in "unauthorised" protests against the Sunni Al Khalifa monarchy, his lawyer says.

Mohammed al-Jishi said on Thursday that Nabeel Rajab was given one year for each of the three cases all related to participating in peaceful protests.

"It is a very stiff and unexpected ruling, I am surprised. They are peaceful protests, not violent ones," Jishi said. 

The verdict is likely to undermine a US call for dialogue to defuse political tension on the island.  

Bahrain, in turmoil since pro-democracy protests led by its Shia Muslim majority erupted last year after successful revolts in Egypt and Tunisia, is a US ally and is home to the US Navy's Fifth Fleet.

Rajab, a Shia, has led many protests during the uprising against the wide powers of the Sunni Al Khalifa dynasty that rules the island kingdom.

He had been held in custody since June 6 for allegedly insulting the Sunni community on social-networking platforms.

Critical of prime minister

Rajab, who serves as president of the Bahrain Centre for Human Rights, has made repeated calls on Sheikh Khalifa bin Salman Al Khalifa to step down as prime minister.

He has also been accused of insulting residents of the mostly Sunni Muharaq province, claiming they only welcomed the prime minister’s visit to their city because he had offered them subsidies.

The Shia community has been buoyed by Rajab’s courage for greater rights in Bahrain and continues to stage sporadic demonstrations in villages since last year's crackdown.

Leading international human rights groups have criticised Bahrain over last year's crackdown on demonstrations that were inspired by the Arab Spring uprisings.

The Paris-based International Federation for Human Rights (FIDH), for which Rajab serves as deputy secretary general, condemned the jail term.

Souhayr Belhassen, the FIDH president, said: "Arbitrarily imprisoning human rights defenders will not stop the people from aspiring to freedom and democratic change."

"We hope that the international community will firmly condemn this decision and will call for Nabeel's release," she added.

Nearly 20 US politicians have supported the campaign for Rajab's immediate release.

Rajab's lawyer said he would appeal the court's decision.