Mauritanian police clashed on Monday with anti-slavery activists demanding the release of an imprisoned legislator as the country's newly-elected National Assembly held its maiden session.Police using truncheons and teargas dispersed demonstrators who had gathered outside parliament in Nouakchott to call for the release of anti-slavery campaigner and new MP Biram Ould Dah Abeid.Abeid is president of an unlicensed NGO called the Initiative for the Resurgence of the Abolitionist Movement.He has been in custody since early August after a journalist filed suit against him for "slander, insults and incitement to hatred.""Nine people were injured," including Abeid's wife, a spokesman for the protestors, Hamadi Ould Lehbouss, said."These are young people who came to demand the release of the MP and leader who has been unjustly imprisoned," he told AFP. "People are having fun ignoring the case and holding him in prison in defiance of any reason."* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookRemnants of traditional slavery have become a major issue in Mauritania, a deeply conservative, predominantly Muslim state. Under a generations-old system of servitude, members of a "slave" caste are forced to work without pay, typically as cattle herders and domestic servants, despite an official ban that makes slavery punishable by up to 20 years' jail.No official figures exist for those still enslaved, but some NGOs estimate that up to 43,000 people remained in bondage in 2016, accounting for around one percent of the population.Inside parliament, the first session of the National Assembly elected last month got underway with the approval of Cheikh Ould Baya as speaker.Baya is a former colonel and ally of President Mohamed Ould Abdel Aziz, whose second and final term of office ends next year.Baya, best known outside of Mauritania as his country's chief negotiator on fishing quotas, was approved by 118 votes to 27.Abdel Aziz's Union for the Republic was the big winner in legislative, regional and local elections held on September 1 and 15, with an Islamist party coming a distant second.Abeid, the runner-up in 2014's presidential election, also won a seat.