Sydney Maree, the black native of South Africa who thought he had cleared all obstacles to international competition after winning the 1,500 meters in the U.S. Track and Field Championships last month, was abruptly denied an opportunity to run in the 1,500 against Britain's Steve Ovett Wednesday night at an international meet in Milan.

Maree's lawyers, Stuart Ross and Skip Masback of Washington, say they fear the rejection may be only the first in a pattern intended to keep Maree, considered to be one of the world's two or three best milers, out of top-level competition in that event on the European circuit this summer.

They said an invitation to Maree to run in the mile at an international meet in Lausanne, Switzerland, has also been withdrawn, but that the meet promoters had said he could compete in other events.

"They are keeping Sydney from running against the best in the world and proving that he is as good," said Ross.

Ross and Masback, whose brother, Craig Masback, is running with Maree in Europe this summer, said Maree had been told that Ovett and his British track rival, Sebastian Coe, had been warned not to compete against Maree this summer. The Milan meet promoters told Maree they needed Ovett in the 1,500 and that Maree would have to run in another event, the lawyers said.

They said Ovett's agent had told Maree that a nonbinding, advisory opinion from the British Amateur Athletic Board suggested that Ovett and Coe could become the targets of "pressure and adverse consequences" from the South African Non Racial Olympic Committee (SANROC) should they compete against Maree.

Efforts to reach the British Amateur Athletic Board by telephone were unsuccessful yesterday. SANROC, most of whose members are expatriate black South Africans, is a firm opponent of South Africa's policy of apartheid.

Maree was cleared for international competition by the International Amateur Athletic Federation (IAAF) this year. He graduated this spring from Villanova University and is a member of the U.S. team for the World Cup Games in Rome, Sept. 4-6.

Maree had previously been barred from international competition because of his South African citizenship. Since 1964, the International Olympic Committee has barred South Africa from the Olympics because of its apartheid policies, and since 1976 the IAAF has threatened sanctions against any athlete who competed against a South African.

But last December Maree married an American, and he has applied for American citizenship. His lawyers maintain there is no longer a valid basis for considering him a South African.

He does not use a South African passport, they said, although since he is not yet an American citizen he has no American passport.Instead, he travels on American-issued permits, which permit him to reenter the country and list him as stateless. To secure passage in and out of the countries where he is scheduled to run this summer, his lawyers obtained visas from the appropriate embassies in Washington.

In addition to the meets in Milan and Lausanne, Maree had been scheduled to run this summer in Dublin; Viareggio, Italy; Zurich; Berlin, and Cologne, as well as in the World Cup.

Denied the opportunity to run in the 1,500 in Milan, Maree chose to compete in the 3,000, which he won.

He is 24 years old, and will not be eligible for full United States citizenship for three more years. By then, his peak running years will probably have passed, his lawyers said.

"He has already sat out the best years of his life. It would be a great tragedy if he could not compete this summer," said Ross.