The Chicago police facility Homan Square was becoming the focus of an organized protest movement this weekend, as the hacktivist collective Anonymous and organizers associated with the Black Lives Matter movement seized on allegations of unconstitutional abuse at the secretive warehouse.

Mayor Rahm Emanuel, the former top adviser to Barack Obama suddenly facing a runoff for re-election, remained at the political fulcrum of a mounting campaign both on social media and the streets of Chicago, where demonstrations were planned for Saturday outside what coordinated campaigners described as mirroring a CIA “black site”.

ADVERTISEMENT

Organizer Travis McDermott said Saturday’s “Shut Down Homan Square” protest was one of several being planned as far away as Los Angeles.

“Hopefully with the presence we expect to have, that will put a little bit of pressure to say, ‘Hey, look – this isn’t going to go away,” he said.

On Friday night, campaigners associated with the Occupy and Anonymous collectives took to Twitter , Instagram and other social-media platforms with the hashtag #Gitmo2Chicago to decry allegations of what users alternatively labeled as a “secret prison” and “torture soon coming to a city near you” .

Six people and multiple Chicago attorneys came forward to the Guardian this week with detailed accounts of police holding suspects and witnesses for sustained periods of detention inside Homan Square, without public records, access to attorneys or being read their most basic rights – involving what they said included shackling, physical abuse and being “disappeared” from legal counsel and family. The Guardian’s recent investigation into Chicago police brutality began the week before, with a two-part account of the tactics of Detective Richard Zuley, who went from Chicago homicide investigator to Guantánamo Bay torturer .

Related: Chicagoans detail abusive confinement inside Homan Square police ‘black site’

ADVERTISEMENT

The Chicago police department, in its only official statement on the swirling allegations , denied the Guardian’s reporting on Tuesday, without giving specifics. In a report on the Guardian’s reporting published on Friday night, the Chicago Tribune characterized local attorneys’ perception of the statement as “laughable”.

Local and national organizers, meanwhile, have zeroed in on Emanuel, who on Thursday night – two days after being forced into an extended campaign in which policing has been a major issue – made his first statements about the Homan Square row.

“That’s not true,” Emanuel said of the Guardian’s reporting, on the local public television program Chicago Tonight. “We follow the rules.”

ADVERTISEMENT

Emanuel has not responded to detailed questions from the Guardian, sent on Wednesday. Another set of questions sent on Friday, requesting comment on human-rights group requests for access to the site and an elaboration of Emanuel’s comments on Homan Square, did not receive a response despite repeated requests.

Emanuel’s press office has been directing queries on Homan Square to the mayor’s deputy director of communications, Adam Collins.

ADVERTISEMENT

A representative for Jesus “Chuy” Garcia, Emanuel’s challenger in the runoff election, said his campaign was planning to address the Homan Square allegations soon.

Protesters on the ground and online weren’t wasting any time.

The activist group Progress Illinois said several more people were expected to tell their stories of being detained at the west-side holding and interrogation compound at Saturday’s protest.

ADVERTISEMENT

“This rally is being organized to remind the mayor and every politician who has a stake in this runoff that they are answerable to the people,” the group in a statement .

Representatives from the Chicago branch of Black Lives Matter, the movement closely associated with the killings of Trayvon Martin in Florida and Michael Brown in Missouri , said they were backing the Homan Square demonstrations as part of a pattern of long-time Chicago police violence.

“In order to uproot the systemic embedded abuse that has allowed for the creation of such ‘black sites’ like that of Homan Square as well as police torturers like that of Jon Burge, transparent and persistent investigation is the first step of many,” a statement attributed to the group read.

Local civil-rights groups have long sought reparations relating to the notorious practices of Burge, the former Chicago police commander who was released from home custody this month and is estimated to have cost the city upwards of $100m stemming from settlements and judgments in civil-rights cases.

ADVERTISEMENT

A second demonstration – promoted on Facebook with the title “Reparations Not Black Sites” – was set for Monday evening near Emanuel’s office and endorsed by Black Lives Matter Chicago, which was seeking “unrestricted access” to Homan Square.

Another event, in Los Angeles on Sunday, cited the Guardian’s reporting as a spark for further action.

“Some fear this may be a ripple effect,” the event’s Facebook page read, “caused by the National Defense Authorization Act, which allows local police departments as well as the US military to detain Americans indefinitely should the state label you a ‘homegrown terrorist’.”

Politicians from Washington to Chicago called for inquiries by the US justice department and Emanuel’s office into the allegations at Homan Square after the Guardian’s investigation surfaced earlier this week.

ADVERTISEMENT

The local chapter of the National Association for the Advancement of Colored People (NAACP) told the Guardian on Friday that it was still “vetting” allegations as more people came forward to both the Guardian and the Intercept late this week.

“Right now what we are looking to do is establish credibility by getting others who have been affected, and the protest will allow for others to come forward,” said Chicago attorney Billy Mills, who said he found the Chicago police statement on Homan Square to be “vague” and “missing specific details”.

“This will not stand,” Anonymous said in a video message .

Oliver Laughland and Spencer Ackerman contributed reporting from New York

guardian.co.uk © Guardian News and Media 2015