Police fired smoke canisters into a crowd of about 400 demonstrators, including families with children, in Ferguson, Missouri, Sunday night, where they were protesting against the killing last week of an unarmed black teenager by police.
A preliminary private autopsy found that Brown was shot at least six times, including twice in the head.
Police drove into the protest area in armored vehicles and shot smoke canisters at watching media representatives during a protest that had until then appeared to be peaceful.
However, the Missouri Highway Patrol said it was using the smoke canisters to disperse "aggressors" who were trying to infiltrate a law enforcement command post, and that the armored vehicles were deployed to ensure public safety.
The trooper put in charge of tamping swirling racial tensions in this St Louis suburb vowed Sunday to stay "as long as it takes," after one person was shot and seven arrested in fresh violence.
Read: Key events following the death of Michael Brown
Protesters react to the effects of tear gas which was fired at demonstrators reacting to the shooting of Michael Brown in Ferguson, Missouri. (Reuters Photo)
Captain Ron Johnson also apologised for a white police officer's shooting of 18-year-old Michael Brown, which caused long-simmering tensions to spill over for more than a week since his death in Ferguson, home to a black majority.
The US Justice Department announced that a federal medical examiner would carry out a second autopsy on Brown's body, citing the case's "extraordinary circumstances." A first one has already been performed.
"I want to start off by talking to Mike Brown's family. I want you to know my heart goes out to you, and I say that I'm sorry," Johnson said to loud cheering and applause from a crowd of more than 1,300 people at Greater Grace Church in Ferguson.
"I will be here as long as it takes."
Read: Govt declares emergency, imposes curfew in Ferguson
Hundreds more people stood outside the church, unable to enter because there was no more room.
Johnson, who grew up in St Louis, also acknowledged the personal toll the violence has taken on him.
"The last 24 hours have been tough for me," said Johnson, who earlier acknowledged that he had cried after the violence overnight.
"This is my neighborhood. You are my family, you are my friends, and I am you. And I will stand and protect you.
"I will protect your right to protest... I saw people cheering and people clapping, and this is what the media needs to put on TV."
A protester stands in the street after police fired tear gas to disperse a crowd during a protest for Michael Brown, who was killed by a police officer in Ferguson, Missouri. (AP Photo)
Overnight tensions flared anew as police confronted around 200 protesters who defied a curfew.
Police used smoke bombs and tear gas to disperse demonstrators, while in a separate incident a woman was left fighting for her life after being shot.
Local media reported that the curfew ordered by Missouri Governor Jay Nixon would remain in place for a second night Sunday, potentially setting the stage for further confrontations.
Brown's death at the hands of a white police officer has reignited a national debate about law enforcement and African Americans.
The Ferguson police force's muscular response also drew criticism after officers were seen facing down protesters in military-grade armored trucks while toting high-powered assault rifles.
Johnson stressed why resolving the unrest in Ferguson was important to him and other black families.Watch: Tear gas fired at protesters in Ferguson over teenager shooting
Read: US: Turmoil, tear gas give way to hope in Ferguson
"When this is over, I'm going to go in my son's room, my black son, who wears his pants sagging, wears his hat cocked to the side, has tattoos on his arms, but that's my baby," he said.
"And we all ought to be thanking the Browns for Michael, because Michael is going to make it better for our sons, so they can be better black men. So they can be better for our daughters, so they can be better black women."
A demonstrator holds up his hands in a cloud of tear gas as police fire it at a group of people protesting the shooting death of Michael Brown. (AFP Photo)
Calls for police accountability
A manager of a special needs institution who gave her name as Angel said there would only be peace in the community once citizens feel safe and have made peace with the police, who she said must be held accountable for what happened.
"Now that St Louis is standing up and people across the nation are standing up, I think it's noticeable that people are not going to allow things like this to continue to happen, they are going to stand for things," she told AFP.
Social worker Tia Mannierre stressed that "injustice is a big problem in a community."
"We really have to set an example for the youth, we really have to do better for the community to come together and just make sure that these officers are held accountable for their actions because it's not okay for you to just shoot someone in cold blood for absolutely no reason at all," she said.
Brown's family has accused authorities of a "devious" attempt to smear their son's character after police named him as a suspect in the robbery of a Ferguson convenience store and released a surveillance video of the incident.
Read: US crowd after teen's shooting: 'Kill the police'
A law enforcement officer on a tactical vehicle watches after a device was fired to disperse a crowd during a protest for Michael Brown, who was killed by a police officer in Ferguson, Missouri. (AP Photo)
Police said the officer, unaware of the robbery, stopped the teen merely for walking in the middle of the street.
The video's release sparked widespread criticism, with even Nixon saying he was unaware ahead of time and acknowledging that "it made emotions very raw."
The Justice Department, which is conducting a separate investigation into the shooting, had urged the Ferguson police not to release the video, The New York Times reported, citing a federal law enforcement official.
The Brown family's attorney, civil rights lawyer Ben Crump, called Brown's death an "execution."
"In some sense, we've been drawn together today by the misery and the pain of the execution of Michael Brown," said Crump, who represented the family of Trayvon Martin, an unarmed 16-year-old black teenager shot to death in Florida in 2012.
"Ferguson today is a metaphor for abandoned urban America."
Read: Hundreds protest police shooting of unarmed black man in California
(With inputs from Reuters and AP)