Seven suspected black guerrillas were shot to death by South African police in a fierce gun battle outside Cape Town, the latest in a series of clashes that are bringing the black nationalists' armed insurgency into urban areas. Gen. Johan Coetzee, the national police commissioner, said that his men, acting on a tip, thwarted a planned attack on a dozen black officers by guerrillas of the African National Congress. He said his men ambushed the guerrillas and four were shot to death in the five-minute fusillade, while three more were chased into the surrounding brush and killed in gun battles. Witnesses told reporters that one of the black nationalist rebels was killed while trying to surrender and that another, wounded and writhing on the ground, was "finished off" by a white police officer. Police spokesmen strongly denied the charges.