FERGUSON • Protesters who gathered Friday to demand clemency from minor criminal charges in Ferguson provided part of a growing focus on St. Louis County’s fractured system of police departments and municipal courts.

The day before, U.S. Attorney General Eric Holder announced a Justice Department investigation of Ferguson’s use of force, searches, arrests, and “revenue raising on the basis of traffic stops.” He said the probe would extend beyond Ferguson if warranted.

Also Thursday, ArchCity Defenders, a legal charity for indigents, joined a group of St. Louis University law professors and Dean Michael Wolff in a letter calling for changes to Missouri Supreme Court Rule 37 that would let municipal courts go easier on the poor. Wolff is former chief justice of that court.

Several groups rallied outside Ferguson City Hall on Friday, renewing a call for clemency on traffic warrants and cancellation of associated fines — things they say are part of the cause of the discord that fueled riots there last month.

They held signs with messages such as “In Ferguson, one household, three warrants” and “A single mom gets locked up for traffic tickets but Darren Wilson gets paid vacation?”

Wilson is the Ferguson police officer whose shooting of an unarmed black teenager, Michael Brown, on Aug. 9 triggered huge protests and clashes with police reflecting a long-standing regional racial divide.

About 50 people stood peacefully, but loudly, for more than an hour, demanding Mayor James Knowles’ presence.

Four police officers prevented anyone from entering City Hall, saying the mayor was not there. A reporter could not reach him for comment.

“No one should have to make a choice between taking care of their children and making a court appearance for some nonviolent crime,” Derek Laney, of Missourians Organizing for Reform and Empowerment, shouted through a bullhorn.

“Ferguson has made a modern day debtor’s prison for low-income people,” he said. “That cannot continue.”

The proposal to the Missouri Supreme Court suggests a change to instruct municipal court judges to consider an individual’s ability to pay before doling out fines or jail time. It also would allow judges to structure fines according to income.

The criminal code, which guides the circuit courts, already has such provisions, based on a series of state and U.S. Supreme Court rulings, said SLU law professor Brendan Roediger, one of the letter’s authors. “This is a constitutional requirement,” he said Friday. “You cannot incarcerate people because of their inability to pay.”

In a report last month, ArchCity Defenders blasted a municipal court system that it says disproportionately affects black people and the poor. It described how they may be jailed for missing a court date or saddled with snowballing fines.

The report focused on three “chronic offenders” — Bel-Ridge, Florissant and Ferguson — but complained of problems in half of the 60 municipal courts examined.

It pointed out that some of the county’s 90 municipalities issue more traffic ticket warrants each year than they have residents. This revenue stream often forms a major part of their budgets.

Defendants are frequently ordered to pay fines of triple their monthly income, it noted. Some may wait in jail up to three weeks to see a judge. The report said very few can afford lawyers, and counsel is not provided.

Among those rallying on Friday was Qiana Williams, 36, a homeless single mother who said she’s been pummeled by the county’s traffic courts for more than a decade. She owes more than $1,600 in fines and has spent weeks in jail at a time. Afraid to drive and unable to pay, she has struggled to get her daughter to school and quit her own attempts at advanced education.

“I’ve turned myself in. I try to pay what I can,” she said.

It hasn’t mattered. She has still found herself in jail next to thieves and drug dealers.

Williams said she once was assaulted and called police. Crying, she recalled what happened next: “I was arrested on my traffic warrants, and he was let go.”

Roediger said it’s too common a story.

“Ferguson still needs to be about a young man who was killed. I don’t want that forgotten,” he said. “But the reality is it did expose this whole underlying system that has people being locked up over and over again. And that does impact the relationship a community has with the police.”