Demonstrators were arrested at protests over the declaration of a mistrial in the case of a Baltimore police officer charged in the death of Freddie Gray, a black man.

The judge dismissed the jury in the involuntary manslaughter trial of Officer William Porter – the first of six officers to be tried in Gray's death – on December 16 after 16 hours of deliberations during which the jurors were unable to reach a verdict on any of the charges against the policeman.

Baltimore City Circuit Court Judge Barry Williams said an administrative judge would schedule a new trial, but said there would be no court proceedings in the case on 17 December. Gray's death triggered rioting in Baltimore and intensified a US debate on police treatment of minorities. On 16 December, scores of protesters marched through downtown Baltimore following the ruling, chanting "we have nothing to lose but our chains" and "the whole damn system is guilty as Hell." Uniformed police officers took up positions throughout the city, including by the courthouse and police headquarters, and at least two demonstrators were arrested.

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Another group of protesters gathered in Gray's neighbourhood, near where a pharmacy was burned during the rioting, where they expressed disappointment at the outcome. Gray's family and officials, including Baltimore Mayor Stephanie Rawlings-Blake, called for calm, eager to avoid a replay of the unrest that followed Gray's death.

Porter, 26, was charged over Gray's death from a broken neck suffered while the 25-year-old man was transported in the back of a police van. The jury of five men and seven women had said on 15 December that it was deadlocked, but Williams had told them to keep trying to reach a verdict.

Porter, who like Gray is black, was charged for having put Gray in the back of the van without seat-belting him and with being too slow to pass on his request for medical assistance. The officer's attorneys had argued that Porter may have been unaware of department policy mandating that detainees be seat-belted, which was put into place shortly before Gray's arrest.