Western governments have only a few months to prevent the slide of Zaire into violent anarchy by withdrawing their support for President Mobutu Sese Seko, according to his former prime minister and foreign minister, who is campaigning against what he calls the most corrupt regime in black Africa.

Nguza Karl-I-Bond has come here to publicize his warnings of impending disaster for Zaire and to offer what he says are details of Mobutu's personal finances. Nguza says Mobutu took more than $150 million in two years from the Zairean national bank for himself and his family.

Nguza said yesterday that if the West does not withdraw support for Mobutu and use diplomatic pressure to oust him, Zaire faces imminent economic and political collapse. "It will be terrible, terrible. Just like Iran," he said.

Nguza, 43, has lived in Brussels since he resigned as prime minister and left Zaire in April. He is known for strong pro-western views, and says U.S. support for Mobutu will rebound on the West because Mobutu, who seized power in 1965 and has ruled almost singlehandedly since, has lost the support of the army and the people and cannot hold the country together much longer.

In startling testimony to the House subcommittee on Africa this week, Nguza said the situation in Kinshasa, the capital, is ripe for uprisings like the Shaba revolts that shook Zaire in 1977 and 1978, and that peaceful change cannot occur while Mobutu retains power.

"Yet change must occur and must occur soon. For the deterioration is so rapid and the danger is so grave that those who would benefit from the destabilization of my country are anxiously waiting to take advantage of the final crumbling of the present regime," Nguza testified.

The population is suffering misery, and some are starving, he said. He told the committee that Zaireans believe Mobutu is responsible, and are aware that he has U.S. support. Mobutu is recognized as the richest man in Africa.

Later, Nguza spoke of his passionate opposition to Mobutu, whom he served as foreign minister and prime minister and who imprisoned and sentenced Nguza to death. "He is like Idi Amin or Bokassa, with one difference," he said. "He is smart."

Former Ugandan dictator Amin has been deposed, as has Jean-Bedel Bokassa, emperor of the Central African Republic.

Nguza said economic disaster has destroyed support for Mobutu, and said Zaire's ambassadors abroad have not been paid for four months. He said Mobutu is in open conflict with the Roman Catholic Church, an important political force, since two-thirds of the population are Catholics.

Throughout the 1970s, Nguza was in touch regularly with politicians in many western countries, and was regarded as a moderate and a friend. Now he is using that good will in an effort to persuade the West that Mobutu should be asked, in effect, to go into voluntary exile with his considerable fortune, leaving others to try moving Zaire toward a more democratic system and to attempt economic improvements.

Nguza is receiving no official recognition here, and the State Department said this week that it is not interfering in the internal affairs of Zaire. As a result, Nguza is trying to get his message to Congress and the media, as he has had some success doing in Europe.

He has brought with him some staggering figures, previously unpublished, detailing Mobutu's financial transactions. He recalled being informed officially as prime minister last year that $30 million in Belgian francs was being transferred by the Bank of Zaire to the president's personal account, and that state companies had been instructed to sell 20,000 tons of copper abroad privately for Mobutu's benefit.

Nguza has also revealed details of a parliamentary investigation into Mobutu's finances showing that from 1977 to the first quarter of 1979 a total of $150,403,350 in foreign currency was withdrawn from the bank by Mobutu.