Thousands of protesters marched against police violence in US cities from New York to Boston, as troops stood by in Baltimore to enforce a curfew imposed after civil unrest over the death of a 25-year-old black man in police custody. The marches were the latest in a string of demonstrations against racial profiling and police use of lethal force sparked by the deaths of unarmed African-American men in Cleveland; Ferguson, Missouri; New York and elsewhere in the past year. New York City police arrested at least three people after hundreds of protesters gathered in Union Square and some of them tried to cross barriers to march through the streets. Boston, Washington and Minneapolis saw smaller demos. In Baltimore, thousands of peaceful marchers converged on city hall capping a day of calm in a city that two days earlier saw its worst rioting in decades. Protesters in the mostly black city sought answers about the fate of Freddie Gray, who died after suffering spinal injuries while in police custody. Police are due to give their findings on Gray's death to prosecutors today but they have said no information about the case will be made public. "Can't stop, won't stop, put killer cops in cell blocks," chanted protesters in the biggest march in Baltimore since Gray died on April 19, a week after his arrest and injury. Republican Governor Larry Hogan said protesters must respect the night time curfew, and that troops would not tolerate looting or rioting. Nineteen buildings and dozens of cars, including police patrol vehicles, burned in Baltimore earlier this week in a spasm of violence which led to several people being arrested.