Demonstrators stand in front of the police line on the corner of W North Avenue and Pennsylvania Avenue, a day after it was looted and set ablaze in protest for the death of 25-year-old black man Freddie Gray who died in police custody in Baltimore, Maryland April 28, 2015. [Photo/Agencies]

The riots started on the Monday afternoon at a mall in downtown Baltimore, where hundreds of police officers were confronted with the protesters and were pelted with water bottles and rocks.

Some fifteen police officers were wounded by rock throwing, several of whom suffered serious injuries, and nearly 20 stores and structures were burned during the riots, local police department said, adding that they were looking into the death of Gray and would release the result on Friday.

On Tuesday, most of local schools were closed and public transit was shut down in parts of the city.

Maryland Governor Larry Hogan declared a state of emergency and called out the National Guard. More than 3,000 police from Maryland, New Jersey and the District of Columbia, and National Guard members in helmets, took up posts in front of businesses and hospitals in Baltimore.

In addition, Baltimore Mayor Stephanie Rawlings-Blake has declared a citywide curfew beginning 10 pm Tuesday (0200 GMT Wednesday). Almost a quarter of people in Baltimore live below the poverty line and decayed, crime-ridden areas of the city inspired the gritty television police drama "The Wire."