On the main road into this southern Zaire city, two men sat on a log, occasionally eyeing two battered tin barrels in the middle of the road, most of the time dozing in the tropical sun or chatting to ambling passers-by.

The men were soldiers of Zaire's army. The barrels represented the only roadblock into Kolwezi, the vital mining center just 50 miles from the strongholds of Katang an rebels from the Congo National Liberation Front.

Inside city limits, small groups of soldiers sat idly on a few street corners, American or Belgian rifles lying against their legs. Three posts had shallow foxholes dug to reinforce their positions.

The scene generally was not an impressive reflection of how to defend a key city against what the Zaire government calls a major invasion.

As a Belgian engineer with property in Kolwezi commented after a tour of the town: Ah, its an African war. What can you expect?

"If the Katangans get really close, many of these soldiers will run anyway. All it takes is a loud bang, and off they go." He shrugged.

The main army activity since the first attack on four cities in southern Shaba Province March 8 has been at the local army headquarters, a house in a posh Kolwezi suburb now run by the paunchy, spirited Zaire army chief of staff, Capt. Gen. Bumba Moaso Djogi.

In a recent interview, Gen. Bumba said there were 10,000 Katangans and mercenaries invading his country. The generally accepted outside estimate of the number of invaders is 1,500. He alleged that his troops had killed many Cuban and Soviet soldiers - identifiable by the markings on their firearms. He added that his forces had captured several tons of weapons from the opposition.

The arms, which he was unable to describe, had been left in place for collection at some later date, however, and there had not yet been time to retrieve the enemy bodies, the general said.

At a hospital in Mutshatsha, northwest of here, there were about two dozen wounded soldiers who said they were shot during, a skirmish a few miles up the road. A doctor at the hospital said he had treated between 60 and 70 persons since March 8. He added that the more seriously wounded soldiers were evacuated to Kolwezi in helicopters.

In light of the army chief's victory claims and sharply differing intelligence reports, the Zaire government of President Mobutu Sese Seko has cause for great concern. The loss of even part of mineral-rich Shaba Province, which supports the rest of this central African country, would be a disastrous economic blow with grave political implications.

Defense of the town's two strategic areas, the airport and the vast Gecamine mint complex, was almost non-existent. There were only two old American armored personnel carriers at their airport, but no bunkers, fences, hangers for Macchi and Mirage fighter-bombers, or sandbag reinforcements for protection against artillery, mortar or rocket attack. Piles of Spanish bombs and American rockets were left in clear view outside the two-room air terminal.

The only visible security at the Gecamine mine - the richest single mineral deposit in Zaire, which produces cobalt, copper, manganese, gold, and other minerals - was the usual complement of unarmed company guard.

The mine, so hugg that it looks like Grand Canyon, is believed to be the next major target of Katangan guerrillas.

Gecamine produces 958,000 tons of copper ore a month in its five open pits, according to director Claude Renard. This is 75 per cent of Zaire's copper production, the country's main export.

Business at the mine has gone on normaly since the Katangans first crossed from bases in neighboring Angola 16 days ago, with few of the 460 European employees making contigency plans to leave the area for safer terrain.

"No one would dare to touch us," a Belgian administrator explained." We are essential to whoever governs this area, so we are not worried."

The same is not true of the city, highly vulnerable with minimal visible defense. Katangan guerrillas, who know the terrain well, have so far operated only after dusk, when protection is even more difficult.

Although Gen. Bumba appears to be taking the Katangan rebellion as a major threat, the defense system for this crucial town and its mineral assets does not match his concern.