The State Department yesterday defended its decision to relax trade restrictions with South Africa, while a leading Senate liberal condemned the move as the foreign-policy equivalent of tax exemptions to segregated private schools.

Sen. Edward M. Kennedy (D-Mass.) said that the action was supportive of South Africa's apartheid policy and "the equivalent in our foreign policy of the administration's blatant recent attempt to give tax exemptions for segregation in the United States." The reference was to the administration's decision last month to reverse Internal Revenue Service policy of refusing tax exemptions to private schools that discriminate on the basis of race.

That reversal touched off a political furor, and the administration hastily agreed to push for legislation to close off exemptions for such schools.

The State Department said that the new trade rules, disclosed Friday, were intended to be "less arbitrary from the perspective of U.S. exporters" while still maintaining "a strong symbolic and practical disassociation" from the policy of apartheid.

It added that the new rules, which reverse a four-year-old Carter administration policy blocking sales of even non-military goods to the South African government, are "at least as extensive" as trade controls maintained by other major western nations.

The new regulations do not permit the sale of arms or military equipment, but congressional critics said the changes would permit the sale of non-military equipment that could be converted to military use, citing as an example the sale of large transport planes to civilian airlines.

The State Department said that such sales would be permitted only on condition that the aircraft not be used for police, military or paramilitary purposes, but written assurances of this, "which constituted a paperwork burden both on exporters and on the U.S. government," would no longer be required.

The new regulations are to take effect at midnight tonight, but Congress or the administration may alter them before they become final 30 days later.