Protest marches have sprang up across the United States, and police battled to stifle another night of unrest in Ferguson, ground zero of America's latest racially-charged riots.

Violence erupted in the St Louis, Missouri suburb for a second night, after Monday's decision by a grand jury not to prosecute a white police officer for shooting dead an unarmed black teenager in August.

Windows were smashed at city hall, a police car was set on fire and protesters pelted police with rocks, bottles, chunks of concrete and a Molotov cocktail, St Louis County police said.