The governor of Missouri, Jay Nixon, appealed for residents of the St Louis area to show “tolerance, mutual respect and restraint” on Monday, hours ahead of an announcement on whether a white police officer will be charged over the shooting death of an unarmed black 18-year-old in Ferguson.



Nixon said he met residents in Ferguson “and it is understandable that, like the rest of us, the are on edge waiting for a decision”.

A grand jury reached a decision over whether to indict officer Darren Wilson for killing Michael Brown around noon on Monday. Bob McCulloch, the prosecuting attorney for St Louis County, is scheduled to make the decision public at Clayton County courthouse at 8pm local time.

Nixon, speaking just under three hours earlier, told reporters that police would facilitate peaceful protest but clamp down on any violence.

Speaking alongside the governor, St Louis County executive Charlie Dooley said free speech of protesters would be protected unless the safety of residents is compromised.

“I do not want people in this community to think they have to barricade their doors and take up arms,” he said. “I do not want someone to shoot or accidentally harm someone out of fear. This is not a time to the turn on each other. It is a time to turn to to each other.”

Brown’s death on 9 August was followed by nights of intense clashes between police and protesters, and Ferguson, a suburb north-west of St Louis, is on tenterhooks over the possibility of a repeat.

Nixon said the national guard were on standby to defend “critical facilities” such as fire houses, police stations and utility substations. “This will help free up law enforcement officers to do their jobs effectively,” he said.

Nixon declined to rule out the use of armoured vehicles and teargas against protesters.

Following Brown’s death, several witnesses told media that the 18-year-old had surrendered to Wilson after fleeing a confrontation at the officer’s patrol vehicle. Some said that Brown had his hands up. Wilson is believed to have told jurors that Brown assaulted him at his SUV and then threatened to do him further harm.

Authorities in Ferguson have been braced for more protests over the grand jury’s decision because many protesters said that they assumed Wilson would not be charged.

US president Barack Obama has urged protesters to keep demonstrations peaceful. Several demonstrators have been arrested in the past week during small protests at the police headquarters.

Evidence relating to the shooting was presented to grand jurors at weekly sessions for more than three months. The 12-person panel was considering a range of charges against Wilson, 28, including manslaughter and murder.

Nixon was speaking at a press conference at the University of St. Louis campus, less than five miles from the road in Ferguson where Brown was shot dead by Wilson.

He appeared alongside local clergy and St Louis Mayor Francis Slay who said he expected some locals to “loudly and passionately” express their views.

“Whatever is announced this evening, some people are going to be angry and frustrated. And some people are going to be angry and frustrated about that.

Jay Nixon speaks ahead of the Ferguson verdict. Guardian

“My message to the protesters: we will protect your right to peacefully assemble and to speak your mind.” But he added: “Turning violent or damaging property will not be tolerated.”

Storefronts have been boarded up across the St Louis suburb as the community braces itself for possible unrest. Three hours before speaking, Gov Nixon made a brief appearance at a burger bar on West Florissant Avenue, scene of days of rioting in August.

The barbershop and Family Dollar stop either side of the premises were covered in graffiti-covered plywood. Nixon was surrounded by bodyguards and was escorted to his vehicle after men from a local barber shop emerged to taunt and heckler the governor.

Darrion Lewis, 19, who was in the barbershop, said he planned to protest on Monday evening, in part because of what he said was the “militarised combat” perpetrated by law enforcement, and overseen by Nixon, in the days after Brown’s death.

“It has been like they’re fighting their own people,” he said. “The governor should have come and talked to us.”