Brown family attorney Benjamin Crump announces that Michael Brown that was shot at least six times by police. (Reuters)

Police came under "heavy gunfire" and 31 people were arrested, authorities said on Tuesday, in racial unrest in Ferguson, Missouri sparked by the fatal shooting of an unarmed black teenager by a white policeman 10 days ago.

"Not a single bullet was fired by officers despite coming under heavy attack (on Monday night)," State Highway Patrol Captain Ron Johnson told a news conference, adding that police had confiscated two guns from protesters.

Demonstrations, mostly peaceful but with spasms of violence by smaller groups, have flared since Michael Brown, 18, was shot dead during an incident with a policeman in a patrol car while walking down a residential street in Ferguson on Aug. 9.

Johnson said Molotov cocktails and other projectiles were thrown at officers. He urged peaceful protesters to limit their demonstrating to daytime hours on Tuesday.

Johnson said two people were wounded by gunfire. The condition of the wounded men and details of the shootings were not available. Some of those arrested came from as far away as California, Johnson said.

"This was not an act of protesters," he said. This was an act of violent criminals."

Missouri's governor had lifted a curfew for the St. Louis suburb of Ferguson on Monday as National Guard troops were called out.

The National Guard deployment was the latest step by authorities to end the looting and burning of stores that have punctuated protests and stirred questions about race relations in the United States since the shooting death of Brown.

Governor Jay Nixon, who had declared a state of emergency for the town on Saturday and ordered the streets cleared for a curfew that ran from midnight to 5 a.m., said the National Guard would fall under the supervision of the Missouri Highway Patrol.

National Guard troops could be seen walking on the fringes of the gathering, keeping a distance from protesters.

President Barack Obama said he told the governor the use of the National Guard should be limited, and urged healing, instead of violence. Attorney General Eric Holder will travel to Ferguson on Wednesday, Obama said.

Protesters would not be allowed to congregate on the streets on Monday, the Missouri State Police captain in charge of the scene had told reporters earlier in the day.

"We're going to stage them in this parking lot," Johnson said. "We are going to contain the criminal element."

The Federal Aviation Administration said it had renewed a ban on low-flying civilian aircraft over Ferguson to help law enforcement authorities do their job.

Mark Stafford, a church pastor from O'Fallon, Missouri, said: "They tell you to stand still, then they tell you to keep walking, then they tell you to stand still."

Getty photographer Scott Olson, with cameras around his neck and his hands bound behind him, was led off the street by police. Getty Images said in a statement it stood behind its photographer and was working to secure his release.

"While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police, only serves to raise tensions and stir chaos," Obama told a news conference. "It undermines, rather than advancing, justice."

The president met Holder on Monday to discuss the unrest. Holder said more than 40 FBI agents were canvassing Ferguson neighborhoods in an investigation that included federal and local officials.

"Moreover, at my direction, an additional medical examination is being performed on the body of Michael Brown," Holder said. Results of official autopsies by federal authorities and the county are pending.