* Opposition says Ingabire handcuffed since arrest * Chief prosecutor says no date fixed for court appearance
KIGALI, Oct 19 (Reuters) - Rwanda dismissed on Tuesday allegations it was mistreating an opposition leader arrested last week after she was implicated in a probe into a former rebel commander facing terrorism charges.
Victoire Ingabire returned from the Netherlands to the central African nation to contest presidential elections against incumbent Paul Kagame in August, but was barred from standing after being accused of crimes linked to genocide.
She was detained by Rwandan police on Oct. 14.
The New York-based rights watchdog Human Rights Watch condemned Ingabire's arrest, and her unregistered United Democratic Forces (UDF) party said on Saturday she was being kept in handcuffs and denied a mattress and clean clothing.
"Those responsible for the management of detention facilities in Rwanda always ensure the rights of detainees and she will not be treated differently," Rwanda's chief prosecutor Martin Ngoga told Reuters.
Ngoga said no date had been fixed for Ingabire to appear in court.
Ingabire was initially taken to a police station in the capital Kigali where Major Vital Uwumuremyi, a former commander of the Congolese-based FDLR rebel group, was also being held.
The police said Uwumuremyi had been arrested while trying to slip back into neighbouring Democratic Republic of Congo under fake identity papers and faced charges of forming a terrorist organisation know as the Coalition of Democratic Forces.
On Monday, three opposition parties wrote a letter to the U.N. Security Council, pressing for the release of all "political prisoners" in Rwanda, including Ingabire and Bernard Ntaganda.
Ntaganda, the founding president of the Social Party Imberakuri, was arrested on June 24, 2010, less than two months ahead of the ballot which saw Kagame win a second term by a landslide.
"The vicious cycle of political violence in Rwanda has been due to a lack of a system for a peaceful competition for and transfers of power between the political elites," the three parties wrote. (Reporting by Kezio-Muske David; Editing by Richard Lough and Noah Barkin)
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