A court in Bahrain has sentenced a prominent human rights activist to three years in prison for his role in unauthorized protests.

A lawyer for Nabeel Rajab, who heads the Bahrain Center for Human Rights, said the sentence was based on convictions in three separate cases. The defense plans to appeal the ruling.

Rajab is already serving a three-month sentence after being convicted in July of posting “insulting” comments on the micro-blogging service Twitter.

Protests broke out in Bahrain last year as part of the so-called “Arab Spring” uprisings that swept across parts of the Middle East and North Africa. Majority Shi'ites have led the opposition in Bahrain demanding a greater political voice in the nation's affairs.

Shi'ites account for about 70 percent of the Bahrain's population. Some of the protesters want Sunni rulers to give up their monopoly on power, while others have called for the ruling al Khalifa family to be ousted completely.

Bahrain's rulers have offered a dialogue with the opposition on political reforms. But many protesters say they will continue until the government accepts their demands.

Bahrain is home to the U.S. Navy's Fifth Fleet.