Street fights between northern and southern Sudanese youths swept through downtown Khartoum this morning, killing at least four persons and leaving many more injured.

The clashes, fought with rocks, clubs and knives, took place on the fringes of a demonstration mounted by Moslem fundamentalists and hard-line military commanders pushing for a tougher war against southern Sudanese rebel leader John Garang.

By early afternoon the streets were quiet. But the rioting, the march and the debate leading up to today's violence reflect persistent, deep and ever more public divisions between the civilian ministers and the transitional military council in the broad coalition government that replaced dictator Jaafar Nimeri six months ago.

The loosely structured government, conceived as a caretaker administration until elections planned for the spring, has proved unequal to Garang's military challenge in the south and unable to woo him successfully on the diplomatic front.

Friction and fighting between the mostly Moslem and Arab Sudanese in the north and the primarily Christian or animist black Africans of the south have plagued this country since before its independence. But Garang, a former Sudanese Army colonel, is waging a broader and more successful rebellion than any Sudanese government has ever faced.

Many senior officers in Khartoum, including Defense Minister Osman Abdalla, portray the government's position in the south as increasingly precarious.

Abdalla said yesterday that Garang's recent attacks on the towns of Bor, El Nasir and Yerol amount to the biggest offensive the country has seen since 1955.

Up against Garang's determined effort, which receives extensive support from socialist Ethiopia, the government here is beset by indecision and a lack of clear leadership.

Such key moves as the signing of an interim constitution repeatedly have been announced and then suddenly postponed and there is increasing speculation among Khartoum's influential intelligentsia about the possibility of another coup.

Some civilian politicians in the government say privately that they believe the demonstrations and the new propaganda offensive against Garang are aimed at scuttling any chance for dialogue with the southern leader at the same time these actions help lay the groundwork for a fundamentalist-military takeover.

But civilians with ties to the Army also suggest that a number of young officers are unhappy with the hard-line approach of their more conservative superiors and might make a move of their own.

The buildup toward today's demonstration began with denunciations earlier in the week by members of the military leadership who said Garang, instead of responding positively to an appeal for dialogue by Prime Minister Gizzuli Daffa-Allah, used a unilateral government cease-fire to redeploy and strengthen his sieges of key southern cities.

Several civilian Cabinet members deplored the military's statements about Garang and came out publicly against the march today, warning that it could undermine efforts to start peace talks.

The march went ahead nonetheless, climaxing in front of the armed forces' headquarters with an address by the chief of staff, Gen. Tawfik Khalil.

The march itself remained peaceful, guarded by truckloads of police carrying Kalashnikov automatic rifles in addition to such standard riot gear as shields and batons.

But a few blocks away near the Meridian Hotel, and briefly in the central business district, gangs of youths surged back and forth through the streets, some shouting, "Long live Garang," and others calling for his death.

On one street corner beneath the windows of the hotel, a young southern Sudanese was caught by a rival gang and appeared to be stabbed repeatedly before he could scramble away.