Bangladesh's High Court has declared the registration of Jamaat-e-Islami, the country's main Islamist party, is illegal, banning it from contesting January's general election.
The court made the ruling in the country's capital, Dhaka, on Thursday after a petition was lodged arguing that Jamaat's charter breached the constitution.
"It is hereby declared illegal," said Moazzem Hossain, the chief judge hearing the case amid tight security outside the courthouse in Dhaka.
Shahdeen Malik, a lawyer for the Bangladeshi election commission, which oversees preparations for elections due next January, said the ruling meant Jamaat could not field candidates.
"As a party Jamaat's registration with the election commission is declared illegal, with the consequence that they cannot contest the election as a political party," Shahdeen Malik told AFP.
"The party can still carry on with other political activities. If it amends its charter, to bring it in conformity with the constitution and reapplies for registration, it can be re-registered."
Violence flared amid mass protests after the ruling.
Police said that hundreds of activists blocked a major road and smashed vehicles in Pabna district, northwest of the capital.
Further protests were anticipated throughout the afternoon and evening.
Jamaat immediately appealed to the Supreme Court against the High Court verdict, senior defence lawyer Abdur Razzak said.
The party would be barred from contesting elections if the Supreme Court upholds the verdict.
The ruling comes after a leading Sufi group, which practises Islamic mysticism, filed the public interest litigation in January 2009 seeking to scrap Jamaat's registration.
Secular protesters have long demanded that Jamaat be banned for its role in the 1971 war of independence, during which it opposed Bangladesh's breakaway from Pakistan.
Top Jamaat leaders are being tried for crimes during the war and four of them have been sentenced to death for murder, mass murder, rape and religious persecution in Bangladesh's controversial International Crimes Tribunal, which is not endorsed by the UN and has been criticised by rights groups.
Protests over the verdicts have sparked violence that has left at least 150 people dead during street clashes with security forces, Human Rights Watch said in a report released on Thursday.
Jamaat has said that the trials are a sham aimed at eliminating the party, which is a key opposition force.