Bahraini prominent rights activist, Nabeel Rajab, was acquitted by aan appeals court on Thursday, but he must still serve a three-year term for taking part in protests.

A judicial source said that Rajab had been handed a three-month term on July 9 for alleged insults made on Twitter to members of the ruling party, which sentence was overturned on Thursday.

Rajab will remain in jail, however, after he received a three-year sentence on August 16 for "unauthorized" protests against Bahrain's monarchy.

The head of the Bahrain Centre for Human Rights, Rajab led anti-government protests following a brutal crackdown on demonstrations against the regime in March 2011.

Human rights groups and Western countries slammed Rajab's three-year jail sentence, urging Bahrain to overturn the ruling.

Amnesty International called for Rajab's immediate release, describing his sentence as "a dark day for justice in Bahrain."

EU foreign affairs Chief Catherine Ashton said she expected "that this sentence...will be reconsidered in the appeal process."

The US and Britain also expressed their concern at the sentence, while the Paris-based International Federation for Human Rights (FIDH) also voiced its opposition.