Arrests for possessing small amounts of marijuana exceeded those for all violent crimes last year, a new study has found, even as social attitudes toward the drug have changed and a number of cities and states have legalized its use or decriminalized small quantities.

And a disproportionate number of those arrested are African-Americans, who smoke marijuana at rates similar to whites but are arrested and prosecuted far more often for having small amounts for personal use, according to the study. The arrests can overwhelm court systems.

Dianne Jones, 45, who was arrested in New Orleans in 2014 on charges of having a small bag of marijuana, spent 10 days in jail because she could not put up a $2,500 bond. She was able to get enough money together only after her daughter sold the family’s television set at a pawnshop for $200.

Later, when Ms. Jones, who is African-American, was unable to pay court-ordered fees and fines, a judge issued a warrant for her arrest.

With marijuana use on the rise, law enforcement agencies made 574,641 arrests last year for small quantities of the drug intended for personal use, according to the report, which was released Wednesday by the American Civil Liberties Union and Human Rights Watch. The marijuana arrests were about 13.6 percent more than the 505,681 arrests made for all violent crimes, including murder, rape and serious assaults.

The report comes in the wake of the fatal police shooting of Keith Lamont Scott last month in Charlotte, N.C. Mr. Scott, 43, had attracted police attention in part because, the police said, he was smoking marijuana.

The report is the latest study to highlight the disparate treatment African-Americans often receive in the criminal justice system, including disproportionate numbers of blacks who are sent to jail when they are unable to pay court-imposed fees, or stopped by the police during traffic stops or while riding bicycles.

People arrested for marijuana possession or other minor crimes often have a more difficult time finding work, including Cory, 31, who declined to give his last name out of concern his comments could affect his parole. He said in an interview that he had failed to find a job after 14 months in a Louisiana prison for his fifth offense involving a small quantity of drugs.

He said he had been turned down by a fast-food restaurant because of his marijuana conviction, as well as at the restaurant where he worked before his last arrest as a fry cook and dishwasher. “I’ve kind of stopped trying,” said Cory, who is African-American.

Tess Borden, a fellow at Human Rights Watch and the A.C.L.U., who wrote the report, found that despite the steep decline in crime rates over the last two decades — including a 36 percent drop in violent crime arrests from 1995 to 2015 — the number of arrests for all drug possessions, including marijuana, increased 13 percent.

The emphasis on making marijuana arrests is worrisome, Ms. Borden said.

“Most people don’t think drug possession is the No. 1 public safety concern, but that’s what we’re seeing,” she said.

Criminologists say that African-Americans are arrested more often than whites and others for drug possession in large part because of questionable police practices.

Police departments, for example, typically send large numbers of officers to neighborhoods that have high crime rates. A result is that any offense — including minor ones like loitering, jaywalking or smoking marijuana — can lead to an arrest, which in turn drives up arrest rate statistics, leading to even greater police vigilance.

“It is selective enforcement, and the example I like to use is that you have all sorts of drug use inside elite college dorms, but you don’t see the police busting through doors,” said Inimai M. Chettiar, director of the Justice Program at New York University’s Brennan Center for Justice.

African-Americans may also be more apt to face arrest, according to researchers, because they might be more likely to smoke marijuana outdoors, attracting the attention of the police.

The report, which advocates the decriminalization of small quantities of illegal drugs intended for personal use, found that while whites are more likely than blacks to use illicit drugs — including marijuana, cocaine, heroin, methamphetamines and prescription drugs for nonmedical purposes — black adults were more than two-and-a-half times as likely to be arrested.

In terms of marijuana possession, black adults were more than four times as likely to be arrested as white adults in the 39 states in which sufficient data was available, according to the report.

The disparities, the analysis found, persist whether there are few or many African-Americans in a given area.

In Iowa, Montana and Vermont — places with relatively small populations of African-Americans — blacks were more than six times as likely to be arrested on drug possession charges than whites.

In Manhattan, where blacks make up about 15 percent of the population, African-Americans are nearly 11 times as likely as whites to be arrested on drug possession, according to the report.

Ms. Jones, who was arrested in New Orleans with a $10 bag of marijuana, said the warrant for her arrests had been dismissed only after a community group for which she is a volunteer raised the remaining $155 of the $834 she owed the court.

“What happened to me shouldn’t happen to anybody,” she said.