Police arrested about six people overnight after they tried to block a street in a protest calling for a grand jury to charge a white police officer over the fatal shooting of an unarmed black teen in August.
A few dozen demonstrators, some chanting "Indict that cop", gathered outside the city police station late on Wednesday in sub-freezing temperatures.
They were faced by officers in riot gear and the arrests were the first in about a week, suggesting tensions were on the rise ahead of the trial ruling.
The grand jury has been meeting for three months to determine whether police officer Darren Wilson broke the law when he shot and killed 18-year-old Michael Brown in an incident that exposed long-simmering racial tensions in the St. Louis suburb of Ferguson, Missouri.
Weeks of sometimes violent protests followed Brown's death, and businesses and schools around Ferguson are bracing for another wave of unrest, particularly if the grand jury does not charge Wilson. Its decision is expected by month's end.
Missouri Governor Jay Nixon has declared a state of emergency and called up the National Guard to back up local police, a move some activists criticized as heavy-handed. The Ferguson area has seen few protests over the past week, and all have been peaceful.
Police in the St. Louis area have been through conflict de-escalation training since August and activist leaders have also been training potential protesters in non-violent civil disobedience techniques.
Activists across the United States planned to stage their own rallies at federal courthouses from New York to Los Angeles.
The National Action Network, a group founded by Al Sharpton, a longtime New York civil rights activists, said demonstrations would occur regardless of the grand jury's decision, with protesters calling for federal charges against Wilson if he does not face local charges.
There are differing accounts of what happened when Wilson shot Brown on Aug. 9. Some witnesses said Brown had his hands up in surrender, while others described a violent scuffle between the two.
(Reporting by Scott Malone; Editing by Peter Cooney and Crispian Balmer)