Rioters rampaged through the capital of Sudan on Monday, smashing cars and shops in violence that officials said left at least 20 people dead, as news spread that John Garang, a prominent rebel leader and the newly installed first vice president of Sudan, had been killed in a helicopter crash Sunday.

World leaders quickly urged Sudanese factions to carry on the peace process in which Garang, 60, played a major role. His triumphal move just over three weeks ago to the capital, Khartoum, marked the end to a 21-year civil war between Sudan's Muslim north and the largely Christian and animist south -- a conflict separate from that in the Darfur region in western Sudan, where violence continues. The Khartoum government called for three days of mourning, and Garang's longtime top deputy was named to replace him as head of the Sudan People's Liberation Movement.

Garang's widow also urged calm, addressing the public by radio, and asked that the peace process continue. In an interview, she said that despite rumors, there was no evidence of foul play in her husband's death.

"We want to keep his legacy alive," Rebecca Garang said by telephone from southern Sudan. "Keeping the peace is how we can honor his memory."

But across Sudan, where people were beginning to lay down their guns, debate a new constitution and draw up plans to build schools and hospitals after years of war and deprivation, many were asking what impact Garang's death would have on the country's fledgling peace process.

Garang died when his helicopter crashed in bad weather just a few miles from his base, New Site village, in southern Sudan. He was returning from an official visit to Ugandan President Yoweri Museveni at his ranch.

More than a million people celebrated at the historic ceremony in Khartoum on July 8, when the burly and bearded Garang was sworn in as vice president under a U.S.-backed peace deal between the Arab-led Khartoum government and the largely African rebel forces of the marginalized south that had long fought for separation.

On Monday, Sudanese officials said the peace deal must be upheld. Lt. Gen Omar Hassan Bashir, Sudan's president and the man Garang once proclaimed as his sworn enemy, called for calm and said in a statement that the country faced "a difficult test." A funeral was expected to be scheduled at New Site.

U.N. Secretary General Kofi Annan expressed "great sorrow" at Garang's death and urged Sudanese leaders to "continue with the process of reconciliation." Annan said Bashir assured him that he would work "very, very closely" with Garang's movement to advance the peace process. "We should all do whatever we can to ensure that it doesn't unravel," Annan said.

The United States, meanwhile, drafted a U.N. Security Council statement lauding Garang for his role in ending Sudan's civil war and calling on Sudanese people "to honor his memory by restoring peace and calm throughout Sudan."

In Khartoum, however, widespread rioting was reported throughout the day. The airport was closed, and diplomats said they heard gunfire throughout the capital. There were also reports of unrest in areas of southern Sudan.

"There are massive riots here. There's a lot of destruction of property. It has now gone to also burning vehicles, instead of just smashing windows. We received reports that the army now is deploying to get the situation under control," said Col. Bjarne Giske, head of the Joint Monitoring Commission, a U.S.-backed force, speaking from Khartoum.

Some rioters accused the Sudanese government of plotting a coup to kill Garang, shouting, "Killers and murderers!" observers reported.

The mood quieted but remained tense after officials imposed a 6 p.m. to 6 a.m. curfew. A Western diplomat reported that the streets were "empty, very quiet," but that there had been "lots of looting, burning, stoning and some deaths." The diplomat said there was "great despondency" among both southern and northern Sudanese.

In Juba, one of southern Sudan's largest cities, soldiers once under Garang's control started looting Arab-owned businesses, according to television reports. They also demanded the expulsion of thousands of Sudanese army troops. Aid workers in southeastern Sudan reported that at least one person had been killed in rioting.

Sudan's civil war took 2 million lives and left millions more displaced. Under the peace agreement, residents of the south will vote in six years to determine whether they want to secede from the rest of the country. The area is one of the poorest places on Earth; children routinely die of malaria and other diseases, such as guinea worm, because of lack of health centers in 90 percent of the region.

On Monday there were concerns that the north-south peace could collapse, and that the loss of Garang's might also lessen chances of bringing an end to the conflict in Darfur.

"John Garang's death has enormous implications for regional peace and security. He was the linchpin of the north-south deal; he was going to be called upon to play the essential role in bringing the warring parties in Darfur together," said John Prendergast, a Sudan expert with the International Crisis Group, who was visiting the region. "His passing has potentially catastrophic consequences."

Gen. Lazaro Sumbeiywo, a Kenyan who helped mediate the peace talks in Kenya, said Garang's death "means a huge setback . . . a loss of a father and a loss of a real leader."

Some officials from Garang's rebel group said they were worried about fighting within the movement, even though his top deputy, Salva Kiir Mayardit, was named to replace him. Kiir is a charismatic figure who commands most of the rebel forces. He sparred with Garang in the past but had recently stood by his side.

"The death of Garang comes at a very critical time in the peace process. There is already a problem that people might misread or read some foul play into his death," said Peter Adwok, a spokesman for the movement.

Staff writer Colum Lynch at the United Nations and staff researcher Robert E. Thomason in Washington contributed to this report.

Rebecca Garang, right, wife of John Garang, grieves after his death in a helicopter crash. She said there was no evidence of foul play. The former Sudanese rebel leader had become a vice president as part of a peace deal.