An excessive police response to protests in Ferguson, Missouri, over the death of an unarmed 18-year-old earlier this year ran the risk of killing demonstrators and impinged on their human rights, according to a new report by Amnesty International.

The report, by Amnesty observers deployed to monitor the protests, found that the militarised reaction to a small minority of violent demonstrators “impacted the rights of all participating” to freedom of expression and peaceful assembly under the US constitution and state law.

Heavily armed police clashed with demonstrators in Ferguson on successive nights in August after Michael Brown was shot dead by officer Darren Wilson. Teargas, stun grenades and rubber and wooden bullets were shot at crowds to force them to leave the streets.

Noting that the so-called “less-lethal” ammunition that was shot at crowds in Ferguson “can result in serious injury and even death”, Amnesty’s 23-page report said on Friday that “at least two children were treated for exposure to teargas” during the protests.

The report made a series of recommendations to authorities on the policing of protests. It called for any officers responsible for human rights violations during the protests to be “brought to account through criminal or disciplinary proceedings as appropriate, and provide full redress to victims”.

The human rights organisation said the use of heavy-duty military-grade equipment by armed officers at demonstrations, which was sharply criticised during the height of the Ferguson crisis, serves to intimidate protesters and “can actually lead to an escalation in violence”.

“Equipping officers in a manner more appropriate for a battlefield may put them in the mindset that confrontation and conflict is inevitable rather than possible, escalating tensions between protesters and police,” said the report.

Amnesty also called for urgent research into the use by police of sound cannons, which emit loud sirens that can cause “temporary pain, loss of balance and eardrum rupture, to permanent hearing damage”. Such a device was used nightly by police in Ferguson attempting to disperse crowds of protesters. Amnesty observers reported feeling nauseous from the sound of the weapon.

Their report went on to say that forcing a mass demonstration to disperse must always be “a measure of last resort, when violence occurs or there is an imminent threat of violence”, noting that only a small number of protesters had thrown items such as bottles at officers.

“The police should not intervene aggressively simply in response to the actions of a small number of participants,” said the report. “Assemblies are always diverse gatherings, and participants do not lose their individual rights simply because a small number of people are behaving violently.”

Amnesty raised concerns over the use of a series of tactics by police in Ferguson, such as a midnight curfew that was imposed after several nights of clashes. The report said that the curfew impacted not only upon the rights of demonstrators but also “the freedom of movement of the general public”, who were forbidden from being on the streets of the St Louis suburb.

The report also criticised the so-called “keep-moving rule” that was imposed inconsistently by police from the night of 18 August. Dozens of protesters were arrested for standing still on the street or sidewalk for more than five seconds. A federal judge in St Louis earlier this month ruled that the tactic was unconstitutional and ordered authorities to stop using it.

The arrests of at least 19 journalists covering the protests was also of serious concern, the Amnesty observers reported. They also suggested that a practice by police officers widely witnessed at the protests of removing identifying markings from their uniforms and declining to give their names when asked during arrests risked violating international law.

The organisation made a series of recommendations to authorities in Ferguson and the department of justice. It said that law-enforcement chiefs must ensure that all policies and training of police for protests complied with international standards and human rights obligations. The organisation also called for the US Congress to pass a proposal that would halt the transfer of equipment from the US military to police departments.

A grand jury in St Louis is considering evidence gathered by county investigators for possible criminal charges against Wilson for killing Brown. The US department of justice and Federal Bureau of Investigation have also been conducting a separate inquiry.

Amnesty said that a “thorough, transparent, independent and impartial” process should be concluded as quickly as possible. It also expressed concern that Missouri’s law on the use of lethal force by police, which gives officers unusually broad latitude, “may be unconstitutional” and was out of line with international standard, which it said generally sanctions the use of deadly force only to protect life.

Steven Hawkins, the executive director of Amnesty International USA, said that “accountability and systemic change” must follow the actions of police “armed to the teeth, with military-grade weapons”, which he described as “excessive force”.

“What Amnesty International witnessed in Missouri on the ground this summer underscored that human rights abuses do not just happen across borders and oceans,” Hawkins said in a statement. “No matter where you live in the world, everyone is entitled to the same basic rights as a human being – and one of those rights is the freedom to peacefully protest.”