BARCELONA, Spain, April 15, 2018(AP) — Thousands of Catalan separatists are rallying in downtown Barcelona to demand the release of high-profile secessionist leaders being held in pre-trial detention. Sunday's demonstration is organized by pro-independence grassroots groups, the National Catalan Assembly and Omnium, whose presidents are among the nine separatists in prison awaiting trial on their role in last year's failed breakaway bid by the northeastern Spanish region. Spain's two leading labour unions support the protest despite the complaints from some members who don't want secession for Catalonia. The secession movement in the wealthy region has plunged Spain into its deepest institutional crisis in decades. Pro-independence parties retained a slim majority in the regional parliament after an election in December.