Darren Wilson left the force citing fears for the safety of local residents and fellow police officers, according to a letter published by local media yesterday.
Wilson wrote in his letter that "it is my hope that my resignation will allow the community to heal."
A grand jury decision on Monday not to charge Wilson in connection with the August 9 shooting death of 18-year-old Michael Brown touched off days of sporadic violence in Ferguson as well as demonstrations in several major cities across the United States.
"It was my hope to continue in police work, but the safety of other police officers and the community are of paramount importance to me," 28-year-old Wilson's letter said.
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Dozens of demonstrators again gathered outside Ferguson police station despite his decision to quit, with protesters setting fire to a US flag and jeering at police officers.
One man, who gave his name as "Kenny," said: "I'm mad as hell. Every night we're going to be here."
Earlier yesterday, a core group of about 100 marchers, many from other US states, set out on a 192-kilometre "Journey for Justice," heading from Ferguson -- a mainly black suburb -- to the Missouri state capital Jefferson City.
The marchers, organized by the National Association for the Advancement of Colored People (NAACP), are demanding the sacking of the Ferguson police chief, nationwide police reforms and an end to racial profiling.