Police in Chicago have spent $1m on riot-control equipment in the last few months ahead of next month's Nato summit, which is expected to attract thousands of anti-war protesters.

Protesters from a coalition of organisations including unions, anti-war and Occupy groups are expected to descend on the city. National Nurses United, the largest nurses' union in the US, is providing free buses to Chicago for activists from across the country even as its own plans to demonstrate were vetoed by the city of Chicago on Tuesday.

While protesters insist demonstrations during the Nato conference – the main action is planned for Sunday 20 May – will be peaceful, police appear to be leaving nothing to chance. Records show that since it was announced the Nato conference would be held in Chicago, police have purchased improved riot gear for both officers and horses. Officers are also preparing to use the controversial long-range acoustic device, or LRAD, during the operation.

Both the G8 and Nato conferences were originally to be held in Chicago, but the White House announced in March that the G8 summit would be moved to Camp David, the heavily guarded presidential country retreat in Maryland. The splitting of the events has led to two separate protests, with some activists planning to protest close to the G8 meeting in Maryland before making the 11-hour journey to Chicago for the Nato conference.

Chicago police confirmed to the Guardian that they will have a LRAD available at the 20 May protest, "as a means to ensure a consistent message is delivered to large crowds that can be heard over ambient noise".

"This is simply a risk management tool, as the public will receive clear information regarding public safety messages and any orders provided by police," said Chicago police spokeswoman Melissa Stratton.

However while the device can be used simply to transmit voice messages, it also has a "deterrent tone", emitted at a volume which is painful to the human ear and which can be used to disperse crowds.

LRADs have been purchased by the US army and navy, and have also been used in commercial shipping as an attempt to drive away pirates. The device was first used at a protest in the US at the G20 Pittsburgh summit in September 2009, however there are ongoing complaints that its use there caused some people to suffer permanent damage.

Karen Piper, a university lecturer, claims she suffered irreversible hearing damage that day, and is currently bringing a legal case against the city of Pittsburgh. "This is a device that has the capability to inflict permanent hearing loss on people," Piper's lawyer, Vic Walczak, told the Guardian, adding that the device is "more dangerous than a Taser".

"We don't believe it should be used against demonstrators. It should not be used outside the battlefield."

In addition to the LRAD Chicago police will have some $1m of new equipment on hand for the protests, including more than 11,000 new face shields which fit onto helmets and "riot gear" to protect horses. The city of Chicago's procurement services website shows that in March $757,657 was spent on 8,513 "retro-fit kits" to be fitted to police helmets. In February 673 of the same kits, which include a face shield and ear and neck protectors, were purchased for $56,632.

John Beachan, the Chicago co-ordinator for the anti-war group Answer, is involved in organising the protest against the Nato conference. He criticised the police spending as "quite striking. It shows what they'd prefer to spend money on."

"People are coming out to protest Nato because they want wars to end and they want the money spent on public services, not on bailing out the banks and not on more wars," said Beachan, a 44-year-old community college lecturer.

"Why is it that the state is spending so much money on arming the police here supposedly in response to what is being planned as a peaceful protest?"

The protest in Chicago will begin with a rally before protesters march to the McCormick Place convention centre. Beachan said the "main demands" were for the US and Nato to leave Afghanistan, "for no war in Iran, and money for jobs and education and not for war".

"This will be the largest demonstration in the country in the spring, without a doubt. It'll be the largest anti-war protest [in the US] in a number of years. We're expecting thousands of people to come to Chicago to protest."

The action has been planned by the Coalition Against Nato/G8 War & Poverty, or CANG8, a loose organisation made up of several activist groups. Micah Philbrook, a member of Occupy Chicago's press relations team, said the members of the occupation would be involved in protests against Nato's presence, one of which will involve shutting down Boeing's Chicago headquarters in protest against government defence contracts awarded to the company.

"We feel that money could easily be spent in our communities, keeping our health clinics open and our schools and libraries that are currently being cut by America."

Philbrook said protesters from Occupy Wall Street and occupations in Minneapolis, Indiana, Austin and Boston had already arrived in Chicago ahead of the Nato conference. He said Occupy Chicago had set up a "housing working group" to find accommodations for people who had come into the city, with some 200 people having arrived over the last week.

Hundreds more protesters will be travelling to Chicago free of charge after National Nurses United said it would pay for bus journeys, more than 12 buses due to travel from cities across the country, including Oakland, Los Angeles, Portland, Detroit and six buses from New York. Almost 700 people have signed up for free places on the buses on the 99solidarity.net website.

On Tuesday NNU had its permit for a demonstration against the Nato summit revoked by the city of Chicago, after the group's addition of guitarist Tom Morello to the event apparently spooked the department of transportation.

Michael Simon, an assistant commissioner of the department of transportation, wrote that the city was changing its mind "to accommodate changes to your event, including a performance by a nationally known musician and a significantly increased number of expected attendees", according to the Chicago Tribune.