Speaking at a news conference at the UN headquarters in New York on Wednesday after meeting Ban Ki-moon, the UN secretary-general, she said: "I am very concerned that we continue to see detention without trial and with, in my opinion, insufficient judicial supervision.
"I thought there had been progress in that direction.
The first prisoners arrived at the US military base in Guantanamo Bay, Cuba, more than five years ago, after the September 11, 2001, attacks, and about 395 are currently detained there.
Arbour said: "I hope that we will see the American judicial system rise to its long-standing reputation as a guardian of fundamental human rights and civil liberties and provide the protection to all that are under the authority, control and, therefore, in my view jurisdiction of the United States."
Her criticism referred to the Military Commissions Act, approved by the US Congress last year, and last month's federal appeals court ruling that Guantanamo Bay detainees cannot use the US court system to challenge
The US appeals court ruling dismissed hundreds of cases filed by foreign-born detainees in federal court and also threatened to deny court access to millions of foreigners legally in the US as permanent residents.
The case is likely to go to the supreme court.
At issue is the right of habeas corpus, a basic tenet of the US constitution protecting detainees from unlawful imprisonment.
Twice before, the US supreme court ruled that right gave Guantanamo detainees full access to courts.
But last June, the justices suggested George Bush, the US president, could ask Congress for more anti-terrorism authority, prompting passage of the commissions act.
The act grants suspects at Guantanamo Bay the right to confront the evidence against them and have a lawyer present at specially created "military commissions".
But it does not require that any of them be granted legal counsel and specifically bars detainees from filing habeas corpus petitions challenging their detentions in federal courts.
Arbour said: "There's been a legislative setback now recently in my view, a judicial decision that again does not provide the scope of judicial review that I would like to see in cases of people facing ... no charges but facing very serious suspicions or allegations."
Arbour said she was an admirer of the American judicial system but stressed that "getting access to court with appropriately supported legal advice and so on is absolutely critical".
She said checks and balances by the different branches of government are also essential to democratic government.