Four leading opposition and rights figures are being held in Niger prisons following a banned weekend demonstration that turned violent in the capital Niamey, media reports said on Tuesday.They were among 23 people arrested after the protest on Sunday against new taxes degenerated into clashes between demonstrators and police.Officials of the Collectif d'ONG, an association of grassroots groups that organised the protest, said those in custody had been charged with organising and taking part in a banned march and "abetting damage to public and private property".Local television said opposition figure Nouhou Arzika, rights activists Moussa Tchangari and Ali Idrissa and rights lawyer Lirwana Abdourahamane were all in custody in different prisons around the former French colony.Rights watchdog Amnesty International called Monday for the 23 arrested to be released."The wave of arrests of activists and the clearly brutal response by security forces not only violates the right of demonstrators to peaceful assembly but also risks inflaming an already tense situation in Niger," Amnesty said on its website.Niger, one of the world's poorest nations, has seen regular protests against the new taxes and tax breaks to telephone companies that critics say are worth tens of millions of dollars.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. Interior Minister Bazoum Mohamed told AFP that Sunday's protest had been banned for "security reasons", notably because it was planned for after dark."They incited (the public) and disrupted public order and were arrested for rebellion for describing the ban as illegal," he said.The Labari radio and television network owned by Idrissa was also ordered closed on Sunday.The economy of the largely desert country has been hit by falling prices for oil, which it officially began exporting in 2011, and uranium, of which it is a major exporter. The government says it is cash-strapped as it has to spend resources to combat attacks by Boko Haram, whose Islamist insurgency has spilled over from Nigeria, as well as from jihadists, including the Islamic State group, near the border with Mali.Finance Minister Hassoumi Massadou said in February that the 2018 budget would "barely affect" people in the countryside, where more than 80 percent of Niger's 20 million people live.