The lone Republican congressman was clearly angry as he arrived at the press conference here half an hour late last night. Rep. Gerald B.H. Solomon of New York brushed past U.S. Ambassador Robert Keeley and snapped, "Is anybody here from the American Embassy?"

In front of about 20 startled reporters and Africa-watchers, Solomon chewed out Keeley, the senior Amercian ambassador in black Africa, and Rep. Howard Wolpe (D-Mich.), chairman of the House African affairs subcommittee, for allegedly misinforming him on the time of the gathering.

Thus began a bewildering demonstration of what one witness called "American reality"--the chasm between President Reagan's conservatives and liberal Democrats in attitudes toward the Soviet Union and its sympathizers, and differing American perceptions of dictatorships of the right and left.

It was a look at American politicians emulating the Third World, one journalist marveled. Or perhaps an episode of Capitol Hill Goes to Africa.

The differences reflected by the congressmen have been evident before in hearings in Washington, where Wolpe often has been critical of the administration's Africa policies and Solomon has risen to the administration's defense, frequently putting African issues in the context of East-West rivalries. But last night's bickering at the press conference was a rare display by U.S. officials before a foreign audience.

"It was like holding a mark-up session in public," said one congressional aide.

Solomon, Wolpe and six other Democratic members of Congress are on a three-week swing through six African countries.

Virulent attacks by Solomon, a conservative Republican, on the Soviet Union and communism were particularly vexing for the Tass correspondent here, Vladimir Paegelis. He maintained a stoic expression for about an hour and then, in a rare display of emotion for the Soviet news agency, stormed to the head table, grabbed his tape recorder and charged out, asking Solomon to "tell us about the atrocities in Chile and El Salvador."

Solomon, a fiesty ex-Marine and former insurance salesman, responded, "I'll get to that," but Wolpe, a former professor of African studies, and two Democratic colleagues managed to calm the situation.

Solomon had entered the meeting just after Wolpe, head of the delegation, had finished a glowing description of the group's visit to Marxist Ethiopia.

"We could not have been more warmly received," he said of the first visit by a congressional delegation since a socialist revolution nine years ago swung Ethiopia from the U.S. to the Soviet sphere. In a three-hour meeting, Ethiopian leader Lt. Col. Mengistu Haile Mariam "signaled the desire for better relations," Wolpe added.

Solomon, who had not heard Wolpe, then demanded the floor. Mengistu, he said, "called us a bunch of terrorists, said we were exporters of revolution and imperialist aggressors." "No question," he said, "there is starvation and extreme poverty along with the worst human rights violations I've ever seen."

Ethiopia, he said, had suffered "almost a decade of corruption, mismanagement and suppression" and he noted the presence of numerous pictures on the streets of Marx, Lenin and the hammer and sickle.

Rep. Mickey Leland (D-Tex.) acknowledged that "Mengistu did say bad things about the United States but it was the usual rhetoric. What he was calling for was constructive engagement" between Washington and Addis Ababa. Constructive engagement is the Reagan administration's policy of seeking to influence South Africa by establishing better relations.

There were even wider differences between Solomon and his Democratic colleagues over their visit to Zaire, where they indirectly became enmeshed in a serious incident. Five dissident former members of parliament--who were recently released from prison--were beaten and arrested outside the hotel where they had just met with the congressional delegation.

"We were shaken by the incident," Wolpe said. "It left a very bad taste as we experienced the realities of contemporary Zaire." The incident, he added, "will have an impact on Congress." Leland, who witnessed the assault, said, "It was one of the most brutal displays I've ever seen."

The delegation filed a protest with President Mobutu Sese Seko and canceled a lunch. However, the group did hold three hours of talks with Mobutu, centering on the assault. The U.S. Embassy said it was carried out by the presidential brigade, despite Mobutu's disclaimers.

Just the week before, Mobutu had visited the United States and received warm praise from President Reagan, who after talks with the Zairean leader, said, "Close relations between our two countries, based on shared interests and perceptions, will advance the cause of peace and development in Africa." The president also lauded Mobutu for granting an amnesty to his political opponents.

Solomon echoed Reagan, saying he conveyed his "thanks to Mobutu for the role Zaire plays against Soviet expansion." He saw the assault in a different light, saying none of the victims were critically injured. He likened the assault to a football game and said the attackers were provoked by demonstrators who tried to use the meeting to promote an illegal second political party in the one-party state.

Leland said Mobutu "threatened" them in an effort to prevent disclosure of the attack. The congressman added: "The arrogance of Mr. Mobutu about our concerns about human rights was despicable."

Responding to a hostile question about U.S. policy in Central America, Solomon said, "I often hear particular groups criticize the United States and I never hear them criticize the other side." Rep. James Moody (D-Wis.) replied, "I hold my society to a much higher standard than I hold Soviet society."

Ninety minutes into the press conference, Ambassador Keeley asked plaintively, "Are we ever going to get to Zimbabwe?" The host country provided the only area of agreement between Solomon and the Democrats, with all lauding Zimbabwe for its "remarkable progress" since independence in 1980.

They soon fell out again when the subject turned to South Africa, with Solomon praising "constructive engagement" in dealing with Pretoria. Wolpe chimed in, "I happen to think constructive engagement has had very destructive consequences."

When the two-hour imbroglio ended, a veteran diplomat said, "I've never seen anything like it in 12 years in working in southern Africa."