WHAT KIND OF thuggery is going on in South Africa? The government has just arrested more than a dozen leaders of the United Democratic Front, the principal black nationalist organization. At least six of them were charged with treason and will be tried next month in Durban along with eight other UDF figures already in jail. The eight had been detained without charges last year for protesting against the new South African constitution, freed partly as a result of President Reagan's intercession, then promptly arrested and charged with treason.

The South African authorities have not made public the basis of the charges. Their record, however, makes it impossible not to suspect a political purpose. The authorities let the UDF be organized two years ago. It was formed out of hundrds of local black union, civic, church and social organizations. For a white leadership looking to reform the apartheid system, as President P. W. Botha claims to be doing, it made a certain sense to sanction a moderate, nonideological, community-based organization devoted to nonviolence. The need for such a structure became especially evident after blacks overwhelmingly rejected the government's diversionary scheme to divert urban-black political aspirations into local town councils.

Almost from the start, however, the UDF has been severely harassed by a government evidently alarmed by its popularity, momentum and potential as a national political vehicle. And now it has been in effect decapitated, although its decentralized, constitutent nature offers it some possibilities of a continuing life. Whether the government will dare to ban an organization claiming more than a million members may be the next question.

Apartheid seeks to reserve to the white minority authentic national political rights and to spin off blacks to tribal "homelands." But the whites have never known how to handle the millions of urban blacks who do not live in the homelands and who cannot all be relocated, if only because they are needed in the work force. The right and necessary thing would be to accept blacks as permanent South African citizens and to begin arranging with their chosen leaders their passage to equality and freedom. But this the white power structure has so far refused to do. It only toys with the idea of releasing Nelson Mandela, a long-jailed nationalist. It accepts, partly for considerations of international display, a UDF and then loses its nerve and manufactures a "treason" conspiracy against it. Its performance is wretched and shortsighted and can only produce further grief and blood.