Uganda President Idi Amin, wearing a khaki safari suit with rulled-up sleeves and a broad plantation hat, his waist encircled by a pistol belt with a revolver clearly visible in a holster, arrived at the airport here today and did a quick change.

When he entered the conference room of the Arab Socialist Union building little more than an hour later, The Uganda dictator was resplendent in full air marshal's regalia, with rows of ribbons and medals across his ample chest.

Amin, who flew here from Kampala in his American-made executive jet, was the last head of state to arrive before the opening of the 60-nation Afro-Arab summit meeting.

Despite the ill-favor in which the towering, portly Amin is held in the Western world these days, the Epyptians gave him the same welcome they gave other visiting dignitaries: a salute by a red, blue and gold-clad honor guard, a band to play the Uganda anthem, and a firm handshake from Vice President Hosni Mobarak.

Amin seemed to be in a good mood but turned somber later as he addressed the meeting, accusing Israel, Britain and the United States of conspiring against him. His speech was carried live on Egyptian and Ugandan television.

A member of the Ugandan delegation on hand at the airport introduced himself to reporters as Jeff Dixon of radio station WNJR in Union, N.J.

[Dixon is program director of WNJR, an official of the station said, and went to Uganda Wednesday at the invitation of the Ugandan government as part of an eight-member fact-finding mission from the United States.]

Half a dozen black Americans are in Cairo on their way to Uganda as "guests of his excellency," Dixon said. The Ugandan embassy here arranged accreditation for them as official summit conference representatives of the Kampala government.

They and the fulltime Ugandan diplomats quickly gathered around copies of last week's editions of Time and Newsweek, just distributed here today, which both had cover stories about Amin's alleged reign of terror. The Ugandans were not eager to discuss this with the foreign press.

Meanwhile, a United Nations spokesman said in New York that 300 refugees from Uganda have registered with U. N. and Kenyan government officials during the past 11 days and about 400 more have entered Kenya and are expected to seek asylum.