RIOT POLICE FIRED rubber bullets and tear gas to chase away demonstrators in downtown Johannesburg, South Africa, Saturday, triggering skirmishes that left city streets strewn with debris. . . . A MAN CONVICTED of raping and murdering 13 children was hanged Saturday in Tunisia. . . . TWO BOMBS EXPLODED near the spot where the agriculture minister delivered a speech in Bangkok, Thailand, but the official was not injured.