KIGALI, RWANDA, OCT. 13 -- A legacy of communal slaughter and mass flight that this tiny country thought it was overcoming has weighed in with a vengeance in the form of a powerful invasion by Rwandan exiles from neighboring Uganda.

The attack Oct. 1 has rekindled tensions between the majority Hutu people and their former Tutsi overlords, who form the bulk of the invading force, and created growing international concern over possible human rights violations in the government's response to the incursion.

Meanwhile, the war, the newest civil conflict on the long-suffering African continent, has brought an unsettling tenor to life in a country best known for its imposing green hills and remote refuges for mountain gorillas.

Heavy machine guns and assault rifles abound at the international airport, guarded by French paratroopers and Belgian soldiers invited in by the government. Some bivouac in green pup tents beside the runway and others are dug in on the perimeter, while crates of mortar rounds rest on the tarmac by the main passenger terminal. Eerily quiet French-made Gazelle helicopters whisk over the capital, on their way to and from strafing runs in the combat zone in the far northeast.

Downtown, Rwandan armored cars squat at key intersections, where soldiers carrying automatic rifles challenge passersby for identification. Security forces launch repeated house-to-house searches, netting thousands suspected of collaborating with the invaders; in the murky tropical dusk, a fresh batch of prisoners is driven by, nine or so men sitting impassively in the open bed of an army truck, surrounded by troops in green fatigues.

The genesis of the drama lies in the historical animosity between the now-ruling Hutu and the Tutsi, who for centuries reigned through a feudal monarchy complete with nobility and serfs. As independence from Belgium approached, the Hutu seized power in communal massacres in 1959, sparking the flight of tens of thousands of Tutsi.

More anti-Tutsi pogroms erupted in 1963, moving philosopher Bertrand Russell to call the killings "the most systematic human massacre . . . since the extermination of the Jews by the Nazis." Another round of tension in 1973, following extensive massacres of Hutu in neighboring Burundi by that country's still-reigning Tutsi minority, added to the exodus. Today, as many as 250,000 exiles of Rwandan origin endure an essentially stateless status in Uganda, the country's northern neighbor.

One of them, Fred Rwigyema, joined the army of Yoweri Museveni when it consisted of only a few dozen guerrillas in the Ugandan bush, and rose to become a major general as Museveni became Uganda's president. When Rwigyema struck across the border at the beginning of the month, he took with him thousands of well-armed deserters from the Ugandan army who, like him, are of Tutsi origin and chafe at a seemingly interminable exile.

Rwanda's single-party government, led since a bloodless coup in 1973 by President Juvenal Habyarimana, has long maintained that the country's crushing demographic and ecological burdens leave it unable to absorb the exiles -- a contention supported by Western diplomats and development workers.

Seven million Rwandans, nearly all of them peasants, are squeezed into an area the size of Maryland, which has 4.2 million people. In the countryside, cultivation is relentless, with bananas, tea, coffee and vegetable plots extending from valley floors up impossibly steep slopes to summits.

The average farmer's plot shrinks inexorably as one of Africa's highest birthrates doubles the population every generation. Last year the Malthusian circle closed, as a famine in the country's south left several hundred people dead.

The rebels say they want more consideration for the exiles' plight, but emphasize that their fight is one for democracy and against corruption. The government sees a narrower motive. Foreign Minister Casimir Bizimungu, in an address Monday to the foreign diplomatic corps, accused the invaders of seeking "a reversal of history" and a return to "forced labor and feudal servitude" -- open references to previous Tutsi domination.

Such suspicions appear to have at least partly animated the massive security sweep launched after the invasion. After days of speculation on the number held, Habyarimana said at a news conference that 3,000 people had been arrested.

At least 1,200 had been held for up to four days without food under the sun in a stadium here before being moved to prisons, prompting Western embassies to express their concern over the prisoners' treatment. They similarly asked for clarification of reports emanating from rebel-held territory that as many as 1,000 people were massacred by government troops -- an accusation strongly denied by Habyarimana and by other government officials who said rebels in civilian clothing had been killed.

Earlier, the government said ethnicity played no role in the arrests, and that each would be followed by investigations of individual cases. But a variety of Western sources, including diplomats and aid workers, said most of those seized appeared to be Tutsi, who comprise about 15 percent of the population.

According to several analysts, the invasion and subsequent arrests have upset a gradual and ongoing process of ethnic reconciliation effected during Habyarimana's 17-year rule. Although the compulsory identification cards still show whether the bearer is Hutu or Tutsi, and although the government limits Tutsi access to high posts, tension had steadily eased over the years. But with the outbreak of the fighting, said one observer, "If you're a Tutsi, you're now suspect. End of story."

The same observers credit Habyarimana's government with enjoying broad support. There are widespread complaints about corruption and favoritism for people from the president's home district. But these are counterbalanced by admiration for the government's accomplishments in providing health care, schools and agricultural services -- the bread-and-butter issues in rural Africa. "He's certainly very popular in the rural areas," said one Westerner with extensive contacts in the countryside. "People feel he's made a big effort to develop."