UN investigators have accused Sudan's government of "orchestrating and participating" in crimes in Darfur that include murder, mass rape and kidnap.
Two million people have been forced to flee Darfur's conflict
They urged the international community to step in urgently. The team's leader called the response so far "pathetic".
The high-level mission issued its report to the UN Human Rights Council, which is meeting in Geneva.
At least 200,000 people are estimated to have died in Darfur's four-year conflict, with millions more displaced.
The UN team was blocked by the Sudanese government in Khartoum from visiting Darfur, but spoke to refugees and aid workers from the region.
The five UN team members travelled to neighbouring Chad, to where many refugees have fled, and where the war itself is spreading.
The situation is characterised by gross and systematic violations of human rights and grave breaches of international law... [the government] has orchestrated and participated in these crimes
Report of UN high-level mission on Darfur
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There they heard reports backing up well-established accusations of serious abuses in Darfur, including mass rape, abduction, and forcing people from their homes.
"There are gross violations of human rights and international humanitarian law, the government is complicit in those crimes with the Janjaweed militia that it arms and trains," said the head of the mission, Nobel peace prize winner Jody Williams.
The Arab militia known as the Janjaweed have been accused of attacking villagers in Darfur, killing inhabitants and forcing others to flee, while the government provides air support.
The government denies the allegation, and accuses the West of exaggerating the problems in Darfur.
The Sudanese ambassador to the UN, Ibrahim Margani Ibrahim Mohammed Kheir, told the BBC that Sudan was co-operating with the UN, and that the situation in Darfur was improving.
The report is the latest in a series by the UN and other international bodies to document large-scale attacks on civilians, and to demand an international response.
Ms Williams said the atrocities detailed in the report were not new. But the investigators were framing the document in a way that was meant to demand action.
It stated that if a state was failing to protect its own citizens, "then the international community has to step up to the plate and assume its responsibilities," Ms Williams told BBC News.
So far, the international response had been "pathetic", she said.
"There are so many hollow threats towards Khartoum, that if I were Khartoum I wouldn't pay any attention either," she said.
"It is more than a tragedy. It was after Rwanda that people said 'never again', and here we are again... and the world sits by."
The continuing conflict will top the Human Rights Council's three-week session in Geneva.
The UN human rights commissioner Louise Arbour is known to be especially concerned at Sudan's lack of co-operation with the team.
She has described such obstruction as an affront which must be addressed.
European members of the council are thought to be planning a motion of censure against Sudan, but this is likely to be resisted by many African nations.
The UN human rights council is less than a year old.
It was created to replace the widely discredited human rights commission and action on Darfur is regarded as a key test of the council's credibility.
Up to now the council has censured only one country - Israel - for its actions in Lebanon and Gaza, prompting critics to say the new body is just as politicised and ineffective as the old one.