The UN High Commissioner for Human Rights has said more than a million people displaced by the Darfur conflict are living in a "climate of fear".
Arbour said she believes women have been raped
Louise Arbour described refugee camps as "prisons without walls" and said Sudan was "in denial" about the extent of rape in and around them.
She said refugees could not envisage going home because they did not trust the authorities to protect them.
Her comments came at the end of a week-long visit to the remote region.
Up to 50,000 people have been killed since the conflict began in early 2003.
Aid workers in Darfur have said women have been raped or attacked when they have left the camps to hunt for firewood.
While the authorities have admitted there is some rape, they have said the problem is not widespread, accusing the women exaggerating their stories.
More than 1m displaced
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
"I think the government as a whole is in denial about the
scale and the severity of the problem," Ms Arbour said.
"I don't believe that it is credible to believe that the
women who are coming forward are fabricating stories that would
bring this amount of shame and stigmatisation upon themselves," she added.
"There is a credible base of evidence that there is a severe, severe, serious amount of sexual violence that is not
being properly addressed."
She said she would be calling for a strengthening of the international presence on the ground and
for more African Union (AU) monitors.
About 150 AU observers, with 300 troops to protect them, are presently monitoring April's ceasefire, which has been broken 20 times in the past month alone.
"Displaced people cannot envisage returning home because they do not trust the government of Sudan to protect them," said Ms Arbour.
"At best they feel the authorities respond inadequately to their concerns, and at worst that they are in collusion with their abusers, including armed groups and militias generally described as Janjaweed."
Last week, Ms Arbour told the BBC that displaced people had complained that some of the men guarding the camps were Arab militiamen who had been responsible for atrocities.
Ms Arbour is due to report to the UN Security Council next week.
Sudan may face UN sanctions if the situation does not improve quickly.