By Ellen Wulfhorst and Daniel Wallis
FERGUSON, Mo. (Reuters) - Tensions mounted among hundreds of protesters waiting for Monday night's grand jury decision on whether to indict a white police officer in the fatal shooting of an unarmed black teen in Ferguson, Missouri, as National Guard and police moved into position for possible mass arrests.
Not far from the stretch of Ferguson that saw the worst of the rioting after the shooting death of Michael Brown in August, dozens of police and military vehicles parked in a strip-mall parking lot that officials set up as a command center for responding to protests.
Hundreds of protesters gathered outside the Ferguson Police Department, growing increasingly boisterous as the minutes ticked down to a highly anticipated 8 p.m. CST (0200 GMT Tuesday) news conference about the grand jury deciding whether to indict a white officer in Brown's death.
Trying to maintain the peace were a handful of Amnesty International volunteers in bright vests and crowds were being held back by barricades surrounding the precinct.
But a couple of protesters with bandanas hiding their faces rushed across the street and started dismantling the barricades, prompting police to emerge from the building and the crowd to begin taunting them.
Officials urged tolerance and assured residents that the National Guard would provide security at critical facilities like fire houses, police stations and utility substations.
"I do not want people in this community to think they have to barricade their doors and take up arms," said St. Louis County Executive Director Charlie Dooley.
Earlier in the evening, outside the police department, the slain teen's father, Mike Brown Sr., slowed down in a car to chat with demonstrators after hearing that the panel had reached a decision on whether to indict Officer Darren Wilson in the Aug. 9 shooting.
Protester Byron Conley, father of a teen son, said he spoke with Brown about the pain of losing a child and "spending a holiday like Thanksgiving without your son. I could tell what he's going through."
'WE WILL NOT LET THIS SLEEP!'
Tensions have been mounting over the decision, which officials fear could touch off a repeat of last summer's violence and property destruction in the predominantly black St. Louis suburb of 21,000 people.
At a makeshift memorial on the street where Brown was killed, a handful of people gathered on Monday night, some wearing masks in the cold evening where temperatures were predicted to dip to 26 degrees F (minus 3 C).
"We are ready for war. We are ready for war. We will not let this sleep!" said a man wearing a "I Am Michael Brown" hoodie sweatshirt, who identified himself only as D. White, aged 30, and said he lived nearby.
Protests, largely non-violent, have been held regularly in the community, with some evidence of self-policing. In recent days, when a protester threw a water bottle or otherwise acted aggressively, the rest of the crowd screamed: "Agitator! Agitator!" and pointed the person out to police.
"I hope we can do this in a peaceful way. I just don't want no one to look at our little town thinking we're a bunch of wild animals. We're really good people here," said Conley, 51, a black resident who works for a medical supply company.
"I'm not on Mike Brown's side and I'm not on Darren Wilson's side. I'm here for the residents and we just want our town back," said Conley.
A small but growing group of 20 protesters, mostly young men bundled against the chill in winter clothing, their faces covered by bandanas reminiscent of the ones worn last summer to shield against tear gas, banged a large metal wash bucket outside police headquarters. They broke sporadically to toss around a football near a parked car covered with painted exhortations including "Honk 4 Mike."
(Additional reporting by Adrees Latif in Ferguson, Sascha Brodsky in New York, Scott Malone in Boston; Writing by Barbara Goldberg, Eric Walsh and Peter Cooney)