H.T. Smith calls it ''the quiet riot.''

Six months ago, outraged that local Cuban leaders did not officially welcome South African activist Nelson Mandela when he visited here in June, Smith and a group of fellow black lawyers called for conventions to boycott Miami until city leaders apologize.

It was a tactic that hit where it hurts worst. Miami, bloodied and scorched by a series of riots in the 1980s, was used to racial protest. But this was something new-black professionals using sophisticated weaponry to attack the venerable institution of tourism, the source of one-quarter of Dade County`s jobs.

So far a dozen conventions have canceled, costing the city $5 million to $12 million. The American Civil Liberties Union and the National Organization for Women have pledged their support. And the boycotters haven`t yet used their cruelest weapon, a video to be mailed to countries and companies all over the world, comparing Miami to Selma, Ala., in the 1960s.

Not since 1980, when the black neighborhood of Liberty City erupted in violence, have Miami`s racial fissures seemed so deep and unbreachable. Perhaps never have its blacks felt so powerful.

On one side of the dispute are the boycott advocates, led by Smith, 43, a short, wiry, quick-witted Miami native who practices criminal law out of a two-story, peach-colored house in a tree-shaded old neighborhood near downtown.

''This is a matter of dignity and respect,'' Smith said. ''Nelson Mandela meets with Mrs. (Margaret) Thatcher, meets with George Bush, speaks at the UN, gets a ticker tape parade in New York. Then he gets to Miami and the politicians take the welcome mat up and slap him in the face-in front of the world. And you must evaluate this snub in the context of the plight of black Miami for many years.''

On the other side of the standoff is the Cuban-American community, led by Mayor Xavier Suarez, the tall, polished, Cuban-born attorney who until now has had friendly relations with the city`s blacks.

''A boycott of this sort is like a silent scream-a silent scream that is no longer silent, that may have been in the making for many, many years,''

Suarez said.

Suarez is sympathetic to many of the black community`s grievances, and he notes his administration has done much to help that community. But he gets testy about the Mandela controversy.

''There`s a deep philosophical problem here that just came to the fore over the issue of this visit,'' he said.

Miami is the only city in the country with both a Hispanic majority and a large black community. In the last 30 years, blacks have watched with growing resentment as Cuban immigrants ascended to power and prosperity while blacks, who make up 27 percent of the city`s population, remained poorer than almost anywhere else in the nation.

This is also a city of immigrants and the dispossessed, people still passionately attached to their homelands. The affairs of Cuba and South Africa, among others, are hometown business. Mandela, revered by the black community as a champion of human rights, and Fidel Castro, reviled by the Cuban community as an oppressor of human rights, are as real and immediate as any local politician.

So when Mandela appeared on ABC-TV`s ''Nightline'' shortly before coming to Miami and expressed his appreciation for the support of Castro, Yasser Arafat and Moammar Gadhafi, there was an explosion.

The Miami City Commission rescinded a proclamation in Mandela`s honor. Suarez joined four other Cuban-American mayors in signing a letter that criticized Mandela`s remarks as ''beyond reasonable comprehension.'' The feud was on.

Suarez notes that Mandela came to Miami to attend a closed convention of the American Federation of State, Federal, County and Municipal Employees, and that government officials as well as local black leaders were not invited to meet with him. Nevertheless, in December Suarez issued a statement expressing his ''regret that the matter was not handled better.'' He refused to apologize.

Since then, Smith and his group have decided that in lieu of an apology they will settle for government proclamations and city keys to honor Mandela. ''They issue proclamations to groundhogs,'' Smith said. ''The county even gave a key to RoboCop.''

More than that his group wants an investigation into police beatings of Haitians in July, better treatment for Haitian refugees and more opportunities for blacks in the tourism industry.

They also want to strengthen black voting power by replacing the at-large election system with district voting.

Until those demands are met, the boycott goes on, they say.

Among the dozen groups that have canceled conventions are the National Conference of Black Mayors, the National Black Prosecutors Association and the 70,000-member National Alliance of Postal and Federal Employees.

''It`s a war of words, and we`re caught in the middle,'' said Mayco Villafana, spokesman for the Greater Miami Convention and Visitors Bureau, as he sat in the bureau`s luxurious 27th-floor offices overlooking Biscayne Bay. ''We respect what the boycotters are trying to do, but we disagree with some of their tactics.''

The bureau, like the city`s largely Anglo business community, has watched in frustration as Miami`s image takes another public bruising. Both have tried unsuccessfully to mediate.

City leaders minimize the boycott`s economic repercussions, but they worry that the emotional rift created by the action could last for years. Sherrill Hudson, chairman of the Greater Miami Chamber of Commerce, says he sees no end in sight. ''The longer this goes on,'' he said, ''the deeper the lines are drawn in the sand.''