President-elect Ronald Reagan's transition team has tried to exclude the U.S. Embassy here from participation in a key meeting next month between Reagan and President Jose Lopez Portillo of Mexico, according to U.S. and Mexican officials.

The maneuvering has embarrassed Mexican officials dealing with the Jan. 5 visit and irritated local U.S. diplomats, one of whom called it "surrealism." The issue is particularly sensitive because it follows other displays of hostility in the Reagan camp toward U.S. diplomats in Latin America and widespread speculation that Reagan's inauguration will bring a change of U.S. policy in this region.

At one point, the Reagan coordinating team headed by Richard Allen, recently named as Reagan's national security adviser, refused even to invite Julian Nava, the U.S. ambassador to Mexico, or any othher member of his embassy staff to the Ciudad Juarez meeting, Mexican officials said.

"It put us in a difficult spot because we wanted our ambassador to Washington to attend," said one official here. "When we asked them to find another trusted person in the [U.S.] Embassy as a courtesy to us, they said that would probably be difficult."

In the end, the Reagan team agreed to allow Nava to be present in Ciudad Juarez at the time of the meeting but, unlike Mexico's ambassador to Washington, Hugo Margain, Nava will not sit in on the talks, Mexico was told.

One high official said there was an "amazing degree of suspicion and open hostility toward the Carter people" in the Reagan camp. "It's incredible, as if they were public enemies," he said.

Nava, who is away from Mexico City, has not been available for comment.[In Washington, the State Department also declined to comment.]

Joe Holmes, Reagan's West Coast spokesman, stressed that the president-elect's trip to Mexico is strictly unofficial. "I wouldn't assume we'd ask the State Department to do the work," Holmes said. "It's a private visit; we're not in office yet."

[Holmes said it would probably be "inappropriate at this point" to have Nava present.]

Several senior members of the embassy staff here complained that the freeze-out by the Reagan people forced them to turn to the Mexican government for details about the meeting.

Until a few days ago "we were completely cut out," said one outraged senior career diplomat here. "It's changing now. But it's been very difficult for us, having to ask the Mexicans to fill us in. I told Washington it's total surrealism."

The snub of American Embassy officers in Mexico City comes against a background of tension between Reagan transition aides and some U.S. diplomats in the region. Less than two weeks ago, two veteran diplomats, the U.S. ambassadors to El Salvador and Nicaragua, said publicly that Reagan's advisers were undercutting their authority in delicate political situations. Reagan staffers earlier had leaked a so-called "hit list" of ambassadors slated for immediate removal because they were believed to be politically incompatible with the new adminstration.

Nava, a Mexican-American history professor, is a noncareer ambassador appointed by Carter less than a year ago, but he was not on the hit list. Immediately following the Reagan election, he surprised the Mexicans and his own staff by suggesting that he might survive the change of administration.

For several weeks in November, Mexican newspapers carried statements from Nava saying that relations between Mexico and the United States were likely "to improve under the Reagan administration."

Nava, who clearly enjoys his assignment here, reportedly told members of his staff that he believed he had "done a good job" here, had close contacts with California Republicans and stood a good chance of staying on.

The initiative for the Reagan-Lopez Portillo talks came early in December when Reagan aides approached Mexico and Canada saying the president-elect would like to meet the leaders of both neighboring nations before his Jan. 20 inauguration.

Canada replied that Prime Minister Pierre Trudeau would not be able to fit a meeting into his schedule. On his way to Latin America, Trudeau will also stop off in Mexico in January.

Mexico accepted, however, and Reagan wil have one day of meetings with Lopez Portillo. The Regan staff has said he wants to discuss trade and illegal migration of Mexicans to the United States. Lopez Portillo primarily wants to raise the U.S. role in Central America and the Carribean, according to officials familiar with preparations for the meeting.

For some time now the political leadership and the press here have expressed concern about what they see as an escalating hawkishness toward the region coming from Republicans.