Three guns were seized in the latest protest rally in Ferguson, which has become ground zero of a new national debate on police treatment of minorities after Michael Brown, 18, was shot August 9 by a white police officer.
Captain Ron Johnson of the Missouri Highway Patrol said protesters threw glass and plastic bottles of water and urine at police towards the end of last night's protest, prompting officers to intervene and make the arrests.
"Tonight we saw a different dynamic," he said.
He attributed this to a concerted effort by community leaders, activists and clergy to keep the rally peaceful and prevent it from being taken over by what he described as violent "agitators".
Also Read
Fears that the fatal police shooting of a knife-wielding black man might renew tensions failed to materialize, after successive nights of clashes with police in Ferguson.
"Hands up, don't shoot!" protesters chanted, holding their hands in the air in what has become the signature slogan of Ferguson's frustration with its overwhelmingly white police department.
A grand jury is meanwhile to begin Wednesday hearing witnesses to Brown's killing, amid calls for the police officer, Darren Wilson, to be put on trial for murder.
Meanwhile Brown's family was preparing for his funeral, which their lawyer said would take place on Monday.
In contrast to previous nights, rather than firing tear gas head-on into the crowd, police with riot shields and armored vehicles kept a lower profile.
They finally intervened around midnight (0500 GMT), pushing the remaining crowd towards a newly designated public assembly area in a former car dealership.