LUSAKA, ZAMBIA, JUNE 27 -- Vigilantes with automatic rifles patrolled the capital today and soldiers held looters at gunpoint in a stadium to try to stop the worst rioting in Zambia's history. At least 23 people were reported killed.

On the third day of violence over increased food prices, rioting spread south of Lusaka.

President Kenneth Kaunda, facing his most serious challenge since he took power in 1964, vowed to crack down. "The party and the forces of law and order will be tough with anyone, including children, found committing these acts of thuggery," he said in a nationwide address, his first since the violence began.

An overnight curfew imposed in the capital Tuesday was extended to 24 hours. At Kafue, 25 miles south of Lusaka, soldiers herded scores of looters into a stadium and held them at gunpoint, police said.

The violence began Monday, a week after the government more than doubled the cost of Zambia's staple food, corn meal, from $2.79 per 55-pound bag to $6.56.

The increase was part of an economic reform program aimed at reducing government subsidies and making the economy of Zambia, one of Africa's poorest nations, more financially viable. It had been recommended by Western financial agencies and donor nations.

Kaunda, who led the former British colony of Northern Rhodesia to independence as Zambia in 1964, said, "The instigators of this criminal stampede . . . hope they can disrupt the now succeeding recovery program of the party." He vowed that the economic policy would continue despite the protests.

After months of simmering political discontent and demands for democratic reforms, Kaunda in May promised a referendum on the future of Zambia's one-party system. No date has been set for the vote.

Student rioters have burned the flag of Kaunda's ruling United National Independence Party and chanted "Kaunda out!"

Riots erupted in December 1986, after Kaunda announced food prices would be raised. Fifteen people were killed in the unrest before Kaunda called off the price increases.