As many as 10,000 people were set to turn out for a massive rally in the riot-scarred city, CBS Baltimore reported, in what could be the biggest show of people power yet in nearly a week of demonstrations that threaten to spread across the United States.
The Maryland National Guard said on Twitter it had nearly 3,000 soldiers and airmen ready to help "keep the peace" in the city.
The 25-year-old Gray's death at the hands of police has reignited simmering resentment in the United States over law enforcement tactics, particularly in their dealings with the black community.
Demonstrations have erupted daily in Baltimore this week, while there have been rallies in many other major American cities, including New York, Philadelphia and the capital Washington, in echoes of the protests that broke out across the United States last year over the fatal police shooting of an unarmed black teenager in Ferguson, Missouri.
Also Read
"The findings of our comprehensive, thorough and independent investigation, coupled with the medical examiner's determination that Mr Gray's death was a homicide... Have led us to believe that we have probable cause to file criminal charges," Maryland state prosecutor Marilyn Mosby said.
Mosby said Gray "suffered a severe and critical neck injury as a result of being handcuffed, shackled by his feet and unrestrained" inside a moving police van following his arrest on April 12. He died a week later from his injuries.
"We believe that these officers will be vindicated as they have done nothing wrong," said Michael Davey, a lawyer for the city's Fraternal Order of Police, suggesting that prosecutors had succumbed to pressure from the days of demonstrations.