At his first press conference as chairman of the Organization of African Unity today, Ethiopian leader Mengistu Haile Mariam sharply criticized the United States for its alleged intervention in Africa, increasing speculation here about the direction the 50-nation body will take during his year as its chairman.

Col. Mengistu's three-hour press conference was threaded with praise for the communist system and warnings against western, particularly U.S., meddling in African affairs.

"The effort of the United States administration to work against the forces of democracy and socialism is growing from day to day, and it has made it quite clear that it will not allow countries to decide their own destinies," he said.

When Mengistu assumed leadership of the OAU from Kenya's pro-western President Daniel arap Moi as a compromise choice over Libyan leader Muammar Qaddafi, there were hopes that his Marxist philosophy would be tempered by the more moderate stance of other member states when speaking in global forums.

If today's press conference is a barometer, however, of the continent-wide policy until the next summit in Conakry, Guinea, a year from now, this will not be the case. Mengistu clearly signaled that a more strident and anti-western tone will be adopted on global as well as African issues.

Mengistu answered a series of questions on the OAU and Ethiopia that had been carefully chosen from a list submitted beforehand. He criticized the United States for "supporting apartheid, Zionism and racism, which are the universal enemies of mankind." He also described the United States' attitude as "aggressive" and said the Reagan administration's buildup of its Rapid Deployment Force and military bases was working against detente.

Relations between the United States and Ethiopia have been at a low ebb since the U.S. decision to support the Somali government of Mohammed Siad Barre. Ethiopia and Somalia both historically lay claim to the Ogaden, an arid region in eastern Ethiopia populated by nomads of Somali ethnic origin.

A two-year, $4 million arms program to Somalia was passed by Congress in August 1980. The weapons were delivered last summer in response to an Ethiopian incursion into Somalia. At the same time, Ethiopian links to the Soviet Union have been strengthened through arms shipments with an estimated value of about $2.5 billion.