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Protesters marched in the street as hearings for six police officers indicted in the death of Freddie Gray took place nearby at the Baltimore City Courthouse on Wednesday.Credit Gabriella Demczuk for The New York Times

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BALTIMORE — Four months after the arrest and indictment of six Baltimore police officers — including one on charges of murder — in the death of Freddie Gray, an unarmed black man, a judge on Wednesday convened the first hearing in the case.

About 50 demonstrators gathered Wednesday morning outside the downtown courthouse, where Judge Barry Williams of Baltimore City Circuit Court began the first in a pair of hearings on a series of motions by defense lawyers that will determine how the trial will move forward. By 10 a.m., the police had made one arrest.

Lawyers for the six officers have asked the judge to dismiss all charges and to remove Marilyn J. Mosby, the state’s attorney for Baltimore City, as the lead prosecutor, on the grounds that Ms. Mosby — who is married to Nick Mosby, a city councilman — is tainted by conflicts of interest and has made prejudicial statements, including “inciting rhetoric” at a news conference announcing the charges.

The defense lawyers have also asked that the officers be tried individually and that the judge move the trial outside Baltimore, where a jury is less likely to be biased by the intense news media coverage in the city. Judge Williams is expected to consider the first three motions on Wednesday and the change of venue request at a hearing on Sept. 10. Cameras are not allowed in the courtroom.

Activists say they are opposed to all the defense requests and intend to make their views known, at both a morning demonstration outside the courthouse and a protest scheduled for Wednesday evening, after the hearing is over.

“We have three messages that we want to make very clear,” said Sharon Black, a volunteer with the Baltimore People’s Power Assembly, which is organizing the morning demonstration. “One is not to drop the charges, that is the most obvious. Second is: No change in venue. And the third is: Do not remove Marilyn Mosby from this case.”

Ms. Black said more than 400 people had signed up to attend, and the group was running a phone bank on Tuesday evening to ensure a large turnout. “We want to make sure the indictments hold, that the prosecution is strong, that these cops are indeed convicted and that they see jail time,” she said.

Mr. Gray, 25 at the time of his death, was arrested April 12 in West Baltimore, a blighted neighborhood of boarded-up rowhouses. Ms. Mosby has asserted that the officers illegally arrested and shackled him, flouting police rules and standards of decency by loading him into a police van without required safety restraints and ignoring his pleas for help during the ride. He died a week later of a spinal cord injury that, Ms. Mosby has argued, occurred during the ride.

His death opened a deep wound in Baltimore, a majority black city with an African-American mayor — and a long history of tensions between black residents and the police. It set off more than a week of protests, then a devastating night of unrest that prompted Gov. Larry Hogan to send in the Maryland National Guard.

Officer Caesar R. Goodson Jr., the driver of the police van, is charged with second-degree depraved-heart murder — in essence, murder with willful disregard for human life. Sgt. Alicia D. White, Lt. Brian Rice and Officer William G. Porter are charged with involuntary manslaughter. Officers Edward M. Nero and Garrett E. Miller face lesser charges, including second-degree assault.