The Black Lives Matter group has organized protests in Minnesota and California on one of the busiest retail days of the year.
Minnesota protesters temporarily closed the international airport and interrupted Christmas shopping at the Mall of America.
Police said a total of 15 people were arrested at both places, mostly for trespassing or obstructing justice.
The demonstrators are angry over a police shooting of a black man in Minneapolis.
Jamar Clark was shot dead last month by officers responding to an assault complaint, in one of a number of recent incidents around the US. Activists say he was shot in the head while handcuffed.
There have been daily demonstrations in Minneapolis over Jamar Clark’s killing.
Hundreds of people took part in the protest at the Mall of America, one of the largest in North America, where Black Lives Matter organizers promised to congregate in defiance of a judge’s warning that the shopping center’s owners could legally block the demonstration.
Police said it was an “unauthorized demonstration” and at one point described the situation as “very, very dangerous”.
Police and mall security staff were speedily deployed against the protesters, warning that any who did not depart immediately would be arrested. For a while the area surrounding the mall was on lockdown, but most stores were only closed for about an hour.
Separately eight protesters blocked southbound traffic on the 101 freeway near the California city’s international airport were arrested on December 23, the San Francisco Chronicle reported.
Photos of the demonstrators on social media showed them holding a sign demanding justice for Mario Woods, a black man shot dead by police who is suspected of a San Francisco stabbing.
Black Lives Matter is a loosely organized movement which was formed following protests over police killings of black men in Ferguson, Missouri, New York and other cities.
Police use of force against African Americans has been an ongoing issue.
Protests have been held nationwide for more than a year after a series of incidents, some of them fatal.