BALTIMORE: Protesters rampaged through downtown Baltimore on Saturday as the city’s biggest demonstration yet over the death of a young African-American man in police custody turned violent.

More than 1,000 people had joined a peaceful 90-minute rally at city hall demanding justice for Freddie Gray, 25, who died last Sunday from spinal injuries, a week after his arrest in west Baltimore.

But the mood shifted dramatically when scores of protesters moved to the vicinity of the Camden Yards baseball stadium, scene of an evening Baltimore Orioles-Boston Red Sox game.

Twelve people were arrested, police commissioner Anthony Batts told reporters.

“My family wants to say: Please, please stop the violence. Freddie would not want this,” Gray’s twin sister Fredericka said.

She spoke alongside Mayor Stephanie Rawlings-Blake, who said she was “profoundly disappointed” by violence she blamed on “a small group of agitators.”

Local television footage showed a crowd hurling traffic cones, soda bottles and trash cans at police officers, before randomly smashing store windows, looting merchandise and vandalizing police cars.

Targets included a 7-Eleven convenience store, a Michael Kors fashion boutique, a financial services center and a cellphone shop in Baltimore’s landmark Lexington Market.

Street signs were torn down, and one motorist got a rock through her car window, local news media reported. At least one city bus was also struck.

Five police cars were seen by an AFP photographer getting their windows smashed, before riot-equipped police intervened.

Fans at the baseball game, which went beyond the standard nine innings due to a tie, were told to remain in the stadium, “due to an ongoing public safety issue,” before the gates were opened for them to leave.

One police spokesman blamed the trouble on “isolated pockets” of individuals who were believed to have come from out of town.

Later in the evening, dozens of police formed a cordon around the Western District police station, scene of nightly protests since Gray died. NBC affiliate WBAL reported two arrests, witnessed from its news helicopter.

Tensions have been simmering in the blue-collar Mid-Atlantic port city of 620,000 as investigators try to establish the circumstances that led to Gray’s death.