The journalist Florence Hartmann, a former correspondent for Le Monde, has been jailed at the war crimes tribunal in The Hague, the body established to try the criminals she devoted her life to exposing. She was arrested ahead of the verdict handed down to former Bosnian Serb leader Radovan Karadžić.

Hartmann’s lawyer said she was being held in isolation, a situation that will last until at least Tuesday because of the Easter holiday.

In a private phone call from detention she said that she was in the bizarre position of “watching General Ratko Mladić [the accused Bosnian Serb military leader] walking around the yard and associating with other prisoners while I’m locked away in a cage. The outrageous thing was to see the UN and Dutch police kick away women from Srebrenica and survivors from the camps who were trying to protect me from arrest, after all they’ve been through”.

Hartmann was convicted of contempt of court in 2009 for revealing in a book that the tribunal had withheld crucial information on the Srebrenica massacre of 1995 from the nearby international court of justice. The conviction was later upheld on appeal.

She was originally fined €7,000, but that sentence was later converted to seven days in jail after the tribunal claimed the fine had not been paid. In December 2011, France refused a request to extradite her.

Hartmann was approached by UN police on a grassy area outside the tribunal on Thursday, where survivors of the war in Bosnia and victims’ families had gathered to wait for the verdict on the former Bosnian Serb leader Radovan Karadžić. The demonstrators attempted to close ranks around her to prevent her being detained, but later police managed to separate her, bring her into the tribunal building and drag her through the lobby as she shouted protests against her treatment.

After working for Le Monde in Bosnia, Hartmann served at the tribunal as spokeswoman for and adviser to its prosecutor’s office. In her 2007 book, Paix et châtiment (Peace and Punishment), she revealed that documents that proved Serbian complicity in the Srebrenica massacre had been sealed by the tribunal.

Hartmann, who did not reveal the contents of the documents, insisted that those who had survived or were bereaved by the massacre had the right to know of the tribunal’s decision to keep them secret.

Her lawyer, Guénaël Mettraux, described her situation as a disgrace on Friday. “Florence is in solitary isolation, totally segregated on what is called suicide watch, which in practice means that the light is on 24 hours a day and she is checked on every 15 minutes.

“I filed three motions yesterday [Thursday], one of which sought to have her granted early release no later than two-thirds of her sentence, as has been accorded to war criminals convicted by the courts in The Hague and Rwanda. I’ve asked for the same to be done for a journalist, and this would mean her being released on Tuesday.

“But the problem is that there is literally no one at the detention unit who can address my inquiries, and my application will not be addressed until after the Easter break. I asked to speak to the commanding officer, and they told me he was away, I suppose on holiday. When I asked to speak to someone about conditions of detention, they told me to call back on Tuesday.”

He went on: “I was also told that my formal application for her to be desegregated would be treated as a prison complaint. While this might help expedite things, I am deeply concerned that this would prevent the public from knowing of the circumstances of her detention. A journalist is being detained in conditions – isolation, segregation, suicide watch – that were supposed to have been created for war criminals. It is incomprehensible.”

Hartmann’s son, Stéphane, said: “She had the visit of the French consul in the Netherlands on Friday – it sounded more like a courtesy visit. Nothing much done except bringing magazines and some chocolate, which she was not able to keep as it was food. Anyway this was a nice gesture to see her, but nothing done.

“Bear in mind that so-called justice takes holidays, therefore even Monday will be off! We are now in the weekend; therefore, my mother will have to rot in jail all this time. But the little justice that we can bring is to share this story.”

The tribunal’s spokesman Nenad Golcevski told the Observer on Sunday that her lawyer’s complaint at the conditions of detention had been “refused by the President”, Theodore Meron. “She is not in solitary confinement, but she is of course separated from the male prisoners”, he said.

Mr Golcevski said that Ms. Hartmann is able to switch off the light in her cell, but her lawyer pointed out that this is seen as a suicide alert, and a warden immediately enters if she does. Asked why the tribunal was detaining a journalist in cells paid for to keep war criminals, Mr Golcevski said: “I am not able to discuss the decisions of the tribunal liberally”.

Andrew Begg, the legal officer for the UN mechanism for international criminal tribunals – the administrative body for the Hague tribunal – said Hartmann’s situation was “subject to the rules of detention and I would refer you to the spokesperson”. At the time of publication, the spokesman for the tribunal had not responded to a request for comment.

There was no reply from the UN press office in New York.

In a statement on its website, the UN mechanism said Hartmann had been arrested on an “outstanding arrest warrant issued in November 2011 by the appeals chamber of the international criminal tribunal for the former Yugoslavia”.

Additional reporting by Julian Borger