BALTIMORE (AP) — A protest over the death of Freddie Gray, who was critically injured in police custody, started peacefully with thousands marching through downtown streets before the demonstration turned violent and volatile Saturday night. The chaotic scene prompted the first public remarks from Freddie Gray's twin sister, who pleaded for peace at a news conference alongside the mayor. Just before nightfall, groups of protesters marched from City Hall to the Camden Yards baseball stadium, where the Baltimore Orioles played the Boston Red Sox. [...] a smaller "splinter group" looted a convenience store and threw tables and chairs through storefront windows, shattering the glass. Earlier, a group of protesters smashed the windows of at least three police cars and got into fights with baseball fans outside a bar. Fire officials said roughly 1,200 protesters gathered at City Hall Saturday afternoon to protest Gray's death, which has prompted near-daily demonstrations since his death on April 19. Police acknowledged Friday that Gray, 25, should have received medical attention at the spot where he was arrested — before he was put inside a police transport van handcuffed and without a seat belt, a violation of the Police Department's policy.