A court here today overturned an order that had limited the freedom of an antiapartheid activist in the city of Port Elizabeth.

The ruling lifts restrictions imposed 11 days ago on Mkhuseli Jack, a 28-year-old black leader, whose banning 11 days ago had been widely seen as a violation of the government's announced policy of negotiating South Africa's political future with moderate blacks.

The order had been strongly criticized by the country's white business community and by the United States, whose chief diplomat for Africa, Chester A. Crocker, told a congressional subcommittee the ban made a "sham" out of Pretoria's professed interest in negotiations.

The ruling followed a judicial decision Thursday that the government had issued invalid detention orders against 14 political activists in August 1984. Four other detainees -- persons who are arrested and held indefinitely without charge or access to a lawyer -- were released following the ruling.

In both decisions, judges rejected the orders because Minister of Law and Order Louis le Grange had failed to state his reasons for issuing them.

Hundreds are detained annually under South Africa's security laws and nearly 8,000 others were held under the state of emergency that went into effect last July and was lifted earlier this month. While the judicial rulings do not overturn the state's broad powers to detain or restrict those it considers political threats, analysts said they may force the government to exercise its powers more cautiously.

The decision overruling Jack's banning could also affect several other "banned" persons, including black activist Henry Fazzie, 61, who was restricted at the same time.

In an affidavit filed in the Jack case, le Grange said he would review all 12 of the existing banning orders in the light of the ruling.

Jack is president of the Port Elizabeth Youth Congress and a spokesman for the committee that last year crippled the city's white businesses with a five-month consumer boycott. Businessmen had painstakingly negotiated an end to that boycott with black leaders including Jack and Fazzie, and they feared blacks would launch a new one.