Civil rights lawyers filed federal lawsuits Sunday night accusing authorities in Ferguson and Jennings, Mo., of running the equivalent of debtors’ prisons by illegally jailing hundreds of mostly poor, black residents for unpaid debts, many of them from traffic tickets.

Local plaintiffs represented by lawyers with Washington-based Equal Justice Under Law, Arch City Defenders of St. Louis and Saint Louis University School of Law sought class-action status in U.S. District Court for the Eastern District of Missouri, charging that the two cities’ municipal court policies violate the U.S. Constitution, which prohibits jailing those too poor to pay and allows the punishment only for those with means who willfully refuse.

The suits, reported by the New York Times and NPR, are part of an effort by legal advocates nationwide to target what they call racially and economically skewed practices by which the criminal-justice system has increasingly passed costs on to defendants. The practices criminalize poverty, civil liberties groups say, fueling the kind of frustrations that erupted after the fatal police shooting last August in Ferguson of unarmed black teenager Michael Brown.

In a report last year, Arch City Defenders, a nonprofit legal clinic, traced tensions between many low-income black residents and mainly white local authorities in part to policies by many of the area’s decentralized local courts.

More than half the courts in St. Louis County engage in “illegal and harmful practices” of charging high fines and fees for nonviolent offenses and arresting people who do not pay, the group said. The group’s co-founder, Thomas Harvey, cited as typical “poverty violations” driving with a suspended license or expired registration or no proof of insurance.

An October study by the nonprofit group Better Together found that municipal courts in St. Louis and St. Louis County collected nearly half of the $132 million in fines paid statewide, despite being home to fewer than 1 in 4 Missourians. The county collected one-third of the statewide total, with just 1 out of 9 state residents.

More than a dozen small towns reap more from traffic-court fees and fines than from sales or property taxes, each with large black populations, the study found.

In Ferguson, population about 21,000, such revenue was the second-largest source of income, $2.6 million of $20 million collected. Overall, the city issued three arrest warrants per household, with traffic-fine revenue increasing 44 percent since 2011.

Ferguson and Jennings officials could not immediately be reached for comment Sunday night.

“I understand the frustration from the defendants,” John Adams, court administrator for Jennings’s municipal division, said in a Friday interview reported by the Times. “At the same time, the defendants should be responsible and take the step of at least showing up in court. It would resolve so many issues,” he reportedly said.

Authorities from Washington State to Colorado to Ohio have backed away from similar practices amid growing attention from public defenders, law professors and activists, including the American Civil Liberties Union.

In December, the Missouri Supreme Court required local courts to allow installment plans, waive or reduce fines for the indigent, and hold “show cause” hearings before issuing warrants for failure to appear.

State Attorney General Chris Koster (D) targeted 13 county municipal courts with a lawsuit over financial reporting requirements, while lawmakers have discussed measures that would consolidate courts that one said function as “little more than ATMs.”

Ferguson has capped court revenue at 15 percent of the city budget, abolished the “failure to appear” offense and related fines, forgiven warrants for nearly 600 defendants, and eliminated fees to tow vehicles and revoke warrants.

“Debtors’ prisons have no place in society that values equality and justice,” said Alec Karakatsanis, co-founder of Equal Justice Under Law, after the group sued Montgomery, Ala., last year in coordination with the Southern Poverty Law Center.

Karakatsanis’s group alleged that Montgomery required plaintiffs to pay fines immediately or sit in jail to pay off debts at a rate of $50 a day, or $75 a day if they also agreed to perform janitorial tasks. The city agreed to reforms, provided attorneys to municipal court defendants, and created new community-service and hardship provisions for the indigent.