Law enforcement officials say they came under heavy fire in Ferguson, Missouri Monday night in the latest clashes with protesters.
State Highway Patrol Captain Ron Johson said officers did not fire a single bullet despite coming under heavy attack.
They responded with tear gas and stun grenades as they attempted to disperse the crowd.
Johnson said 31 people had been arrested as of 2 a.m. local time. He said some of those were not from Ferguson or the surrounding area, but came from as far away as New York and California.
He urged those who want to continue to demonstrate to do so during the day, to avoid what he called the "dangerous dynamic of the night."
The demonstrations have been going on since 18-year-old Michael Brown was fatally shot by a policeman on August 9.
While many protesters have demonstrated peacefully, there have been outbreaks of violence and looting.
