A MODISHLY DRESSED young man strolled through the lobby of the expensive Kilimanjaro Hotel one recent afternoon wearing a shirt emblazoned with the emblem of Black September, the Palestine terrorist group.

It turned out that he was neither Palestinian nor a member of Black September.He didn't even seem to know much about terrorism. He just liked the shirt.

He was a walking symbol of the split personality of Tanzania's capital, a bustling port filled with Swahili charm where self-proclaimed revolutionaries mix freely with blue-jean clad youth's whose main preoccupation seems to be catching the latest Bruce Lee kung fu movie.

In bookstores, pedestrian tracts on the virtues of armed struggle nestle absurdly against detective thrillers, accounting texts and "Charlotte's Web," by E. B. White.

Along Independence Avenue, bold posters vauting one liberation struggle or another compete for space in shop windows with adding machiners, shoes and hard-to-get space parts.

Near the center of town, rock music shatters the early morning peace from the discotheque of the Tanzania Stars Co'op. Down the street, are the somber offices of the South West African Peoples Organization (SWAPOL), the militant group now struggling to wrest control of Nambia from South Africa.

SWAPO IS not alone. Other groups that have found Dar es Salaam attractive include the Zimbabwe African National National Union (ZANU), its Patriotic Front ally the Zimbabwe African Peoples Union (ZAPU), and the backers of deposed Ugandan president Milton Obote, who hopes to run Idi Amin out of Kampala someday.

The Palestinian Liberation Organization boasts three telephone lines and a separate office in Zanzibar.

Just about any group claiming the title of liberation movement can stake a claim to the sympathies of Tanzanian President Julius K. Nyerere.

Lacking the wealth of a Nigeria or the sophisticated arms catalogues of the East and West blocs, Nyerere usually dispenses as aid the only two things he can afford to give - moral support and hospitality.

Nyerere has turned this capital into a diplomatic crossroads of Africa. The hotels do a good business and the airport bustles, even without the heavy influx of tourists and businessmen on which rival Nairobi, thrives.

IN THE RESORT city Arusha, the huge Mt. Meru Hotel looms over the countryside like a monument. It is parquet floor shines, its plush bar echoes nightly with disco music and its well trained staff shuttles noiselessly about, forever cleaning even the empty ashtrays.

Since it opened in January as Tanzania's newest class-A [Word Illegible] hotel, the Mt, Meru has had a lot of empty ashtrays. And empty rooms.

Nearby, another, an even more rambling editice sprawls across several acres of Arusha. It, too, is new, its glass and stone still awaiting the final touches. It, too, is empty.

It is the headquarters of the East African Community, once the corner-stone of cooperation among Kenya, Tanzania and Uganda. The problem has collapsed, scattering most of the staff who would have filled the building and, more noticebaly, slamming shut the border between Kenya and Tanzania, and keeping out the hotel guests.

Tourists used a stream across the border daily. Today, a Tanzanian border guard explains testify that only vehicles with non-Kenya plates may cross. There aren't many of those in Kenya.

It was Tanzania that closed the border in February, two days after Kenya made the unilateral decision to ground East Africa Airlines, a community venture. Kenya argued that it was carrying the burden of the financial ailing airline's debts, and seized most of the aircraft. Uganda had dropped out long before.

Tanzania's response was to seal the border, trapping hundreds of Kenyan-owned tourist vehicles here.

Tanzania is out a major supplier, several aircraft, and much of its tourist business. By March, tourism, the country's eighth largest foreign exchange earner, was off 45 per cent nationally and 96 per cent in the lucrative northern game-park circuit for which Kenya used to supply the clients.

Tourists hotels in northern Tanzania are begging for customers, just as tourists in Kenya are eager to fill them. Everybody seems to agree that the community, which pooled transport, postal and other services, was the valuable heritage of "much-maligned British colonialism.

PART OF the problem stems from basic differences between the two countries. President Jomo Kenyatta's Kenya is a capitalist haven laden with imported luxuries and a wide disparity in incomes. Tanzania is having a sincere go at President Julius K. Nyerere's African brand of socialism, an odd blend of morality and homegrown African tradition that is sometices clumsily applied by bureaucrats.

Kenyans like to ridicule Tanzania's mistakes and exaggerate them with depressing tales of hardship. They laughingly refer to their own questionable economic success as "Kenyan socialism."

Tanzanians tend to shrug this off, but get bitter about Kenyans taking credit for Tanzania's landscape.

"The tourists didn't even know they were in Tanzania," says a civil servant. "The Kenyans told them this was just another part of Kenya."

A French television crew ran a documentary saying the world-reknowned Serengeti plain was in Kenya. For the record, it is not, and neither is Mt. Kilimanjaro.

No everybody in Tanzania is bothered about closure. Kenyans have been making $100 million a year from tourism, and Tanzania Tourist Corp. chairman J. K. Chande calls the border problem "a blessing in disguise," citing the dollars he may win away from Kenya.

If tourists are the new pawns in the game, Tanzania appears to have the long-term upper hand. It has by far the world's best game parks, good hotels in the middle of them and Kilimanjaro Airport, possibly the largest in Africa and until now used for nothing but local traffic.

Kenya, on the other hand, has Nairobi and some attractive terrain, but poachers have been allowed to scour the Kenyan countryside so thoroughly that world-famous Amboselli game park, once known for rhinoceros, now has gone.