“We have talked about having to manage expectations,” said Brandon M. Scott, at 31 the youngest member of the City Council. “My message is to remain calm, let the law be the law and to help folks understand that when you’re dealing with trials and cases, you are dealing with the legal system. Cases aren’t going to handle emotions or feelings and opinions.”

Local activists said officials were right to be concerned.

“People are waiting for a guilty verdict in these charges,” said Sharon Black, an organizer with the Baltimore Peoples Power Assembly who is helping to plan the demonstrations. “It may be an unpopular view with some, but nothing ever happened in Baltimore until the young people rebelled.”

Legal experts said it was extremely difficult to win guilty verdicts in cases against police officers. And they caution that the Porter trial, which will revolve around narrow legal questions, will not yield the sweeping debate over police brutality that many black residents, who complain of decades of “police terror,” would like to see.

“Freddie Gray’s death raised massive and critical social policy issues, issues about racial profiling, issues about abuse by police,” said David Jaros, an associate professor of law at the University of Baltimore.

But Professor Jaros said issues such as “why Freddie Gray was initially stopped, why or whether police should be able to chase someone down just because they are in a high crime area and run from the police” might not surface.

Mr. Gray was arrested around 8:40 a.m. on April 12, in a run-down section of West Baltimore, after two police officers on bicycles spotted him and he ran away. He was charged with carrying an illegal switchblade, but a lawyer for his family, which eventually settled with the city for $6.4 million, said his only crime was “running while black.” He died in a hospital of a spinal cord injury a week later.

A central question of the trial is how that injury took place — in particular, what happened to Mr. Gray during his ride in the police van. Mr. Gray seemed healthy when he entered the van; when he arrived at Baltimore’s Western District police station 40 minutes later, after several stops, he was not breathing and was sent to the hospital.