Organisers of demonstrations in Ferguson, Missouri, promised to intensify their protests over the killing of Michael Brown if the officer who shot him does not face criminal charges, warning police that they are prepared to die on the streets for their cause.

Three prominent members of the protest movement that sprung up after the deadly police shooting of Brown, an unarmed 18-year-old, told a rally in New York on Tuesday night that there would be a fierce backlash if a grand jury declined to indict officer Darren Wilson.

“If they can’t serve justice in this, the people have every right to go out and express their rage in a manner that is equal to what we have suffered,” said Ashley Yates, a co-founder of Millennial Activists United, who was arrested last week while protesting in Ferguson.

Yates spoke alongside Tef Poe and Tory Russell, activists for Hands Up United and the Organisation for Black Struggle, hours after it emerged that authorities in Missouri were making plans to deal with potential riots in the event of Wilson avoiding prosecution.

“We’re going to take our anger out on the people who have failed us, and if they are prepared to deal with that, then let them have at it,” said Yates. Poe said that while people in America often expected “casual revolution”, Ferguson may be “the moment when we can’t do that”.

More than a week of unrest followed Wilson’s fatal shooting of Brown on 9 August after the city officer stopped the 18-year-old and a friend for jaywalking. Several witnesses have said Brown was shot while fleeing and when his hands were up. Police say he assaulted Wilson.

Police cracked down on the protests with a militarised presence including armoured vehicles and dozens of armed officers in riot gear. Demonstrators were shot with teargas and rubber bullets. More than 220 people have been arrested since the protests began. A grand jury in St Louis county is now considering evidence gathered by a county police inquiry.

The three activists said that in addition to the arrest of Wilson, protesters were demanding that Bob McCulloch, the prosecuting attorney for the county, whom they accuse of pro-police bias, to step down from the case, or for the Missouri governor, Jay Nixon, to remove him. They also want the resignation of Ferguson’s mayor, James Knowles III, and its police chief, Tom Jackson, over their handling of the crisis.

Speaking in the ballroom in Washington Heights where Malcolm X was shot dead in February 1965, they defended protesters’ conduct in previous clashes. Poe criticised what he called the “intellectual set” who analysed race relations but “didn’t show up and didn’t want to get shot when the teargas came out”. To authorities’ claims that he and fellow activists had incited rioting, Poe said: “No, you incited a riot by leaving Mike Brown’s body on the street for four and a half hours.”

They also dismissed a distinction drawn by the police between “good protesters” and so-called “outside agitators” blamed for causing trouble in August. Russell, who was also arrested during the protests, said 20th century civil rights pioneers such as Martin Luther King Jr, were “outside agitators”. He said: “Don’t be afraid to come from outside and help us do this work. Don’t be afraid to be an outside agitator for some true change.”

Asked during a question-and-answer session “are you ready to die for this?”, Poe said: “Don’t come to Ferguson if you aren’t ready to die. Stay at home, as it could happen.” Yates said: “I can say this with 100% certainty: all three of us have had moments in the street where we realised we could die right there.”

Officials from the county police, Ferguson police, Missouri state highway patrol, St Louis city police and FBI have been meeting to prepare for potential unrest, Reuters reported on Tuesday. Knowles said that there were fears that “the unrest is going to be far beyond the city of Ferguson.”

Mike O’Connell, a spokesman for the Missouri department of public safety, confirmed the meetings had been taking place but denied additional reports circulating on social media that the National Guard was also preparing to assist.

Tuesday’s speakers are preparing to take leading roles in “Ferguson October”, a planned three days of protests and civil disobedience this weekend. Yates said a “mass convergence” would descend on the St Louis suburb. “We’re hoping for the same response we got in the first weeks, only with some organisation this time,” said Yates.

O’Connell told the Guardian: “There are plans for this weekend. It’s safe to say that just as there plans for dealing with crowds in Ferguson, it would be routine and expected to have meetings to get ready for this weekend’s protests.” He said the response of law enforcement would be “appropriate and measured”.

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