"Congo's prisons are overpopulated because there are many prolonged detentions as most detainees don't have access to justice," she said. "The great majority of Congolese do not have access to justice and then face major obstacles to benefit from fundamental rights."
Arbour, who visited the Kinshasa penitentiary re-education centre, said reforms to improve the detention system in DRC needed to be accelerated. The day before her visit, the Ministry of Justice had released 252 detainees, saying they had been held on lesser offences and had served more than a quarter of their sentences.
Justice Minister Georges Minsay said their release would also relieve congestion in the prisons.
The penitentiary is the DRC's biggest and has about 3,000 detainees although it was constructed during the colonial times with to hold 1,500 convicts.
Before leaving Kinshasa, the capital, to visit eastern DRC on 16 May, Arbour met President Joseph Kabila, Prime Minister Antoine Gizenga and other government officials, as well as civil society representatives, with whom she discussed human rights violations and the climate of impunity in the country.
"I have to underline the necessity to engage strongly and concretely in the fight against a culture of impunity which probably has to constitute the dominant focus of human protection efforts," she said.
She proposed that the human rights violation inventory cover the period 1993-2003 - before the International Criminal Court (ICC) was set up.
Arbour criticised the appointment of people implicated in war-related crimes to the government. "The UN is against any form of amnesty for war crimes and crimes against humanity," she explained. "If the government does not have the means to proceed with arrests, at least it should not reward them by giving them posts within the security service or the administration."
Many former fighters, rebel chiefs and militia leaders are serving in the DRC army or government, especially in Ituri, northeastern region.
According to her office, the fight against immunity for serious violations of human rights tops the agenda during the High Commissioner's visit, which ends on 25 May and will also take her to Burundi and Rwanda.
In various meetings, she is underscoring the importance of addressing past abuses, including through transitional justice mechanisms, and discussing efforts to establish human rights as one of the essential elements in consolidating peace and sustainable reconstruction.
She is also focusing attention on the need to devote priority attention to economic, social and cultural rights, through increased transparency and the inclusion of different parts of the population and accountability in governance.