(Recasts, adds details on second fatal shooting, Holder, quotes)
By Scott Malone and Ellen Wulfhorst
FERGUSON, Mo., Aug 19 (Reuters) - Police in Ferguson, Missouri, braced for another night of racially charged unrest on Tuesday as the governor and U.S. attorney general appealed for calm in the St. Louis suburb 10 days after the fatal shooting of black teenager Michael Brown.
The predominantly African-American community of 21,000 people has been gripped by street protests punctuated by looting and clashes with police every night since the unarmed, 18-year-old Brown was killed by a white police officer.
The officer, Darren Wilson, was placed on administrative leave pending an investigation and went into hiding as Brown's family and supporters called for his arrest.
In a separate incident miles (km) from where Brown was slain, St. Louis metropolitan police said they shot and killed a 23-year-old black man who brandished a knife at officers and yelled at them to "shoot me now, kill me" on Tuesday afternoon. Two officers opened fire after he refused repeated orders to drop his weapon, police said.
While apparently unrelated to the Brown killing or protests surrounding it, Tuesday's shooting sparked concerns about an escalation of public anger over what many perceive as a pattern of excessive police force against minorities.
"It definitely could" provoke more street violence, said Walter Garrett, 66, a black retiree in Ferguson. "People are not going to keep sitting here being murdered without reacting."
Others questioned why officers in St. Louis failed to use Taser stun guns to subdue the knife-wielding man who appeared to be emotionally disturbed.
Accounts of Brown's slaying differ. According to police, Wilson reported that Brown reached into the policeman's cruiser when Wilson approached him on the street, then grabbed for the officer's gun.
A companion of Brown said the teenager was initially shot after the officer tried to grab him through the car window and again after Brown staggered back from the cruiser with his hands in the air.
An independent autopsy arranged by Brown's family found he had been shot six times, including twice in the head.
At least 57 people were arrested in Ferguson on Monday night and early Tuesday, most accused of disobeying police orders to disperse, during hours of confrontations marked by volleys of tear gas fired at demonstrators and rocks, bottles and gasoline bombs hurled at police.
State Highway Patrol Captain Ron Johnson, placed in command of security in Ferguson last week after local police tactics that were criticized as overly harsh, said officers had come under "heavy gunfire" on Monday night but did not return it.
Several journalists covering the confrontations have been hit by tear gas and arrested.
Some merchants and civic leaders have suggested that the bulk of the looting and vandalism has been committed by thugs or outside agitators bent on goading police into action.
"Our innocent people need for a moment to go into their homes and not allow this criminal element to hide behind them," Johnson told reporters on Tuesday evening.
Similar appeals came in public messages released on Tuesday from Attorney General Eric Holder and Missouri Governor Jay Nixon.
The turmoil in Ferguson, while generating international headlines, has exposed simmering racial tensions in a mostly black town whose police force, political leadership and public education administration are dominated by whites.
It also has reignited a national debate over racial disparities in the U.S. criminal justice system, even drawing sharp words from the United Nations' top human rights envoy, Navi Pillay, a native South African.
"I condemn the excessive force by the police and call for the right of protest to be respected," she said in Geneva.
President Barack Obama said on Monday he had urged Nixon to exercise restraint in his deployment of National Guard troops ordered to assist with security in town. Members of the Guard mostly kept their distance from demonstrators on Monday night.
Holder, who planned a visit to Ferguson on Wednesday to be briefed on a separate civil rights investigation he ordered into the Brown shooting, called for demonstrators to remain law-abiding in an opinion piece published online by the St. Louis Post-Dispatch.
A separate message of conciliation was issued by Nixon, who promised a "vigorous prosecution" in the case and "justice for the family of Michael Brown." On Monday, the governor lifted a curfew he had imposed days earlier to little effect.
STARTING OUT PEACEFUL
As on previous evenings, protests along a major Ferguson thoroughfare that has been the center of tensions started out peaceful but boisterous on Tuesday.
Police stood by as demonstrators marched down the street, chanting: "No justice, no peace," and a man knelt in the road with his hands over his head. Some of the marchers carried single stems of roses, while others assembled in a newly designated area set aside by police for protests.
Also on Tuesday, lawyers for Brown's family announced plans for funeral services to be held on Monday.
Wilson has yet to make a public statement, but investigators said he had been cooperative in interviews with detectives.
Support for the officer was growing in the form of fundraisers and street demonstrations. A Facebook page titled "I SUPPORT OFFICER WILSON," which describes itself as a voice for law enforcement, said Wilson acted in self-defense. A separate Facebook page was offering T-shirts for sale with proceeds to go to Wilson and his family.
The St. Louis County prosecutor's office said it could start presenting evidence to a grand jury on Wednesday to determine if Wilson will be indicted.
The U.S. Department of Justice and the St. Louis County Police Department are both investigating the shooting. (Additional reporting by Lucas Jackson in Ferguson, Carey Gillam in Kansas City, Mo., Eric Beech in Washington and Curtis Skinner in New York; Writing by Carey Gillam and Steve Gorman; Editing by Lisa Von Ahn and Peter Cooney)