Nabeel Rajab, an opposition activist and head of the Bahrain Centre for Human Rights, has been sentenced for three years in prison for taking part in an illegal protest against the ruling family on Thursday.
As the sentence was read out in court, Rajab was heard to shout, "three years or 30, you cannot stop me".
He is also due to face charges of defamation for Twitter posts calling for the resignation of Bahrain's prime minister, Khalifa bin Salman al-Khalifa.
The hearing has been condemned by Amnesty International who described it as "a dark day for justice in Bahrain".