In the wake of a massive reelection victory last July, Prime Minister Robert Mugabe had some words of warning for his political opponents.

Zimbabwe was going to rid itself of "undesirable elements," he said at a press conference, singling out his main black rival, Joshua Nkomo. As for the people of Matabeleland, the southern region that is Nkomo's political base, Mugabe said, "we are satisfied ourselves that without Nkomo, without the dissident element, they will fall in line."

In the six months since, Mugabe has made good his threat. While world attention has been focused on racial violence and a declared state of emergency in neighboring South Africa, Mugabe's government has used its own broad emergency powers to round up several hundred opponents, many of whom were held illegally, mistreated and tortured, according to Amnesty International, western diplomats, local lawyers and human rights advocates here.

Many have since been released, but an unknown number remain in jail, and at least 10 persons, including three senior officials of Nkomo's party, face charges of plotting a coup.

But rather than loosening Nkomo's grip on his home region, the arrests have exacerbated tensions in Matabeleland, where the government says killings and economic sabotage by armed men it calls "dissidents" have increased in recent months. Diplomats in Harare warn that South Africa, which views Zimbabwe as the region's leading black power, could exploit the chronic disaffection of the region's 1.6 million inhabitants -- about 20 percent of Zimbabwe's population -- to undermine political stability here.

The crackdown casts a shadow over Zimbabwe at a time when other prospects look brighter. The country is enjoying a second straight year of booming agricultural surpluses, its infant mortality rate has dropped significantly and there are indications that some whites who fled to South Africa following the advent of black-majority rule in 1980 are beginning to trickle back.

It also has pitted black officials against human rights organizations like Amnesty International that offered support when they themselves were detained without charge during the days of white-minority rule under former prime minister Ian Smith. In denouncing charges of brutality and torture, officials have claimed that these groups are conspiring with western diplomats and journalists to smear Zimbabwe.

"What do they want this government to do -- sit down and fold their hands and just let dissidents run rampant?" asked Enos Nkala, the Cabinet minister in charge of police, in a recent interview.

"We will not do that. I don't care what Reagan says, what the American people say, what your ambassador here says -- they don't run Zimbabwe. We don't have to go to Washington to get instructions as to how to deal with dissidents."

Ever since Mugabe ousted Nkomo from his Cabinet in 1982, the government has conducted periodic security crackdowns in Matabeleland aimed at armed bands who officials contend receive support and aid from Nkomo and his followers. Hundreds of civilians died during the 1983 antidissident campaign, and dozens of Nkomo supporters were abducted mysteriously last year before the election by plainclothes teams widely believed to have consisted of security police.

At the same time, the government says dissidents have killed more than 600 civilians during the past four years, including more than 100 officials of Mugabe's ruling party.

Throughout these previous campaigns, the government said it regretted civilian deaths, blaming them either on dissidents or on "cross fire" between bandits and security forces. But Mugabe sounded a different note in last year's reelection campaign, warning Matabeleland residents that he would interpret a vote for Nkomo's party as a vote for dissidents.

Trouble started the week after the election, in which Mugabe's ruling Zimbabwe African National Union took 64 of 80 black parliamentary seats. Members of ZANU's youth and women's brigades took to the streets of Harare's black suburbs, burning and looting nearly 600 houses of Nkomo's alleged supporters, most of them members of his Ndebele-speaking ethnic minority.

At least six persons were killed. The rioting continued for three days without police intervention and ceased only after senior ZANU officials condemned the violence.

Mugabe broke his own silence a week later. He blamed opposition "infiltrators" for provoking his followers, but deplored the violence as "unfortunate and out of step with party principles." But, he added, "unrepentant" opponents would soon find "things will get tough."

Within days, a new crackdown began. By the end of September several hundred officials and supporters of Nkomo's Zimbabwe African People's Union had been rounded up, including five members of Parliament, the mayor, former mayor and mayor-elect of Bulawayo, Matabeleland's leading city.

The Catholic Justice and Peace Commission, a church-funded human rights organization, compiled names of 207 arrestees. Of these, 187 were Bulawayo municipal employes, including street cleaners, drivers and city policemen. Other informed observers here believe that the actual number of arrests could exceed twice that figure.

Many of those arrested were taken to Stops Camp, a former horse stable that serves as a holding pen inside the Mzilikazi Police Station in Bulawayo. There, according to a November statement by Amnesty International, "many are reported to be beaten with truncheons, rhinoceros-hide whips, rubber hoses or sticks -- often on the soles of the feet, where scarring is less evident. Some victims are said to be hung upside down and beaten, with their heads in buckets of water."

Amnesty also described incidents of electric shock and a form of torture in which canvas bags full of water were bound around victims' heads.

Nkomo's house was raided on several occasions, property confiscated and several of his bodyguards and aides were detained. In a recent interview, he said those arrested had been tortured, as had a ZAPU member of Parliament, Kembo Mohadi.

Many of those arrested were denied access to lawyers or families, according to human rights advocates.

The main architect of the crackdown has been Nkala, 53, whom Mugabe named as home affairs minister in July. Like Nkomo, Nkala speaks Ndebele, but opposes him and is a close ally of Mugabe and a senior member of the ruling Politburo. Nkala was overwhelmingly defeated in the campaign in a parliamentary district in Matabeleland, and his appointment as minister in charge of police was widely seen as a gesture of vengeance against ZAPU supporters.

Nkala reportedly has used the police Internal Security and Intelligence unit to carry out the official crackdown. This is a plainclothes unit believed to report directly to the minister.

Amnesty telexed Mugabe's office in late October reporting its findings and urging him to end incommunicado detention, give clear instructions prohibiting torture and set up an independent inquiry into the allegations. Receiving no reply, it released its statement.

Zimbabwean officials called the allegations false.

Nkala, who spent 14 years in detention under the Smith government and was adopted as a "prisoner of conscience" by Amnesty, met with organization officials in London last November. He described them in the interview as "little babies who are trying to make a name for themselves out of denigrating this country and denigrating me . . . . "

Western diplomats contend that Nkala visited Stops Camp after returning from London, discovered torture had occurred there and ordered a cleanup. But the minister called that account "silly." He said he found no evidence of torture.

In arguing for a renewal of the 20-year state of emergency last month, Nkala told Parliament that dissidents had committed 103 murders, 57 rapes and 263 armed robberies during the past six months. Among the dead have been five whites, an indication that the insurgents again are targeting white commercial farmers.

The violence seems to have intensified since Nkomo consented to hold a new round of unity talks between his party and Mugabe's. The talks have stalled over a number of issues, and it is unclear whether Nkomo, 67, retains enough influence to persuade people in Matabeleland to accept a political deal, even if one were struck.

Zimbabwe long has contended that South Africa operates training camps for dissidents in the northern Transvaal, and many diplomats here have gradually come to believe the charge, which South Africa denies.

So far, the government has presented no new evidence that South Africa is aiding the new round of dissident activity. Most of those captured or killed in recent months have been poorly armed, according to military sources. But the potential for South African involvement remains.

Northwest of Bulawayo, residents speak of the appearance of small bands of young men whose relatives were killed or harassed in the first government crackdown. Ill-fed and poorly armed, these bands are said to make up in brutality what they lack in equipment.