BLOOMINGTON, Minn. — A mass of demonstrators chanting “Black lives matter” converged in the Mall of America rotunda on Saturday as part of a protest against police brutality that caused at least part of the mall to shut down on a busy day for holiday shopping.

The group Black Lives Matter Minneapolis had more than 3,000 people confirm on Facebook that they would attend. Official crowd estimates were not immediately available, but pictures posted to social media by local news organizations showed the rotunda was full. The organizer of the demonstration, Mica Grimm, estimated that about 3,000 people participated.

The Mall of America increased security, and certain parts of the mall were closed for some time. Signs were posted at some entrances advising shoppers that the east side of the mall was on lockdown.

There were no official reports on arrests, but Ms. Grimm said she knew of at least five people who had been arrested.