A prominent Bahraini human rights activist has been convicted of instigating and participating in several anti-government rallies and sentenced to three years in jail.

The unexpectedly harsh sentence against Nabeel Rajab is likely to raise questions about the Western-backed Sunni monarchy’s commitment to reform, and embolden anti-government protesters who have been demonstrating for the past 18 months, calling for greater rights in the Gulf island kingdom that is home to the US Navy’s 5th fleet.

The monarchy considers most anti-government rallies as “illegal gatherings” punishable by law.

Rajab, who is already serving a three-month sentence for posting anti-government comments on Twitter, was in court for the verdict. He is the president of the Bahrain Centre for Human Rights.

Rajab’s lawyer Mohammed al-Jishi said his client was sentenced to a year in prison for each of three protests he took part in, bringing the total sentence to three years. Mr Al-Jishi said he plans to appeal.

Bahrain has experienced near-daily protests since February 2011, following an uprising by the kingdom’s Shiite majority seeking greater political rights from the Western-backed Sunni monarchy. At least 50 people have died in the unrest and hundreds have been detained.