Demonstrators chanted and sang in front of a security guard who wouldn't let them past the lobby after they had marched from the federal courthouse where a lawsuit challenging the law was filed Wednesday, the Detroit Free Press reported.
The crowd dispersed about two hours after gathering outside City Hall, where they sang "We Shall Overcome."
Earlier in the day, the Rev. Charles Williams II, a protest organizer, charged Michigan Gov. Rick Snyder's appointment of Kevyn Orr as emergency manager of Detroit was unconstitutional and a violation of voting rights.
His statement echoed the allegations made in a lawsuit filed Wednesday.
The lawsuit, which seeks an injunction preventing the law from going to effect, charges the law "disproportionately impacted" Michigan's African-American population because it is used primarily in predominately black communities, The Detroit News reported.
The law gives new powers to emergency managers, including authority to immediately amend budgets, reject or modify contracts and approve new contracts and consolidate or eliminate departments.
Opponents contend the law takes away the rights of citizens to hold elected officials accountable.