By Ian Simpson BALTIMORE (Reuters) - Several Baltimore police officers were injured on Monday in violent clashes with young people after the funeral of a black man who died in police custody, and local law enforcement warned of a threat by gangs. Youths stoned a city bus and passing police vehicles outside the Mondawmin Mall in northwest Baltimore just a few blocks from the site of the funeral of 25-year-old Freddie Gray. Some demonstrators carried signs reading "Justice 4 Freddie Gray." Gray's death has led to days of protests in the latest outcry over U.S. law enforcement's treatment of minorities. Several police officers were injured, Baltimore Police said on an official Twitter feed.