BALTIMORE — A largely peaceful protest over the death of Freddie Gray, a 25-year-old black man who suffered a spinal cord injury in police custody, gave way to scattered scenes of chaos here on Saturday night, as demonstrators smashed a downtown storefront window, threw rocks and bottles and damaged police cruisers, while officers in riot gear broke up skirmishes and made 12 arrests near Camden Yards.

Shortly before 10 p.m., Mayor Stephanie Rawlings-Blake convened a news conference at City Hall, where she appeared with several others — including Mr. Gray’s twin sister, Fredericka; a prominent pastor, Jamal Bryant; and City Councilman Brandon Scott — to appeal for calm. By that time the disturbances had largely settled.

Mr. Gray’s sister, appearing composed less than 48 hours before her brother’s scheduled funeral, spoke only briefly, saying, “Freddie Gray would not want this. Freddie’s father and mother does not want the violence.”

Hours earlier, a racially diverse and mostly calm crowd of hundreds — and by some estimates more than 1,000 — marched through the streets, clogging intersections, carrying signs and shouting, “All night, all day, we’re gonna fight for Freddie Gray!” They made their way from the Gilmor Homes — the squat brick public West Baltimore housing development where Mr. Gray was arrested on April 12 — through the sparkling downtown harbor, a major tourist attraction here, before assembling on a plaza at City Hall.