The South African government sent a combined force of 7,000 police and Army soldiers into three black townships south of Johannesburg today in the biggest crackdown on political dissent ever mounted in this country.

The heavily armed men, backed by armored vehicles, threw cordons around the segregated townships, which have been the scene of racial unrest for nearly two months, then moved from house to house searching an estimated 225,000 inhabitants. By late tonight 363 people had been arrested, mostly for "minor crimes," a police spokesman said.

It was the first time the South African authorities have used mass searches to seek out dissidents whom they regard as political "agitators." It was also the first time the Army has been employed so openly to quell racial unrest.

Louis Le Grange, minister of law and order, said the operation -- codenamed Palmiet (Bullrush) -- was launched to "rid the area of criminal and revolutionary elements."

Black political movements denounced the operation as "an act of aggression equivalent to civil war" and warned that it would lead to increased racial bitterness.

The scale of the raid and the overt use of the Army, which was not called in during the more extensive Soweto disturbances of 1976, apparently reflect an anxiety on the part of the South African government that the current wave of unrest is becoming politically threatening.

The unrest began as the government inaugurated a new national constitution in early September that gives limited political rights to the people of mixed race and Asian origin but continues to exclude the 73 percent black majority.

Resistance to the new arrangement has given rise to two big new black political movements, the United Democratic Front and the National Forum, which are expressing African nationalist sentiments not heard in South Africa since the government outlawed the main nationalist parties in the early 1960s and mid-70s.

Coinciding with this has been a series of protest demonstrations against higher rents for state-owned houses in the townships, which led to clashes with the police.

In the two months since the constitution was inaugurated, the two issues have become fused into one, and the disturbances, though still at a lower level than in 1976, have spread over a wide area.

The government suspects the new political movements of trying to discredit the new constitution and destabilize the country during a transition period. It has threatened to ban the United Democratic Front, the bigger and more effective of the two movements, but realizes that to do so would damage the reformist image it is trying to project.

Caught in this dilemma, it appears to have decided on a show of force to try to stamp out the trouble in the three townships where it began Sept. 2 and has continued most persistently -- Sebokeng and its smaller neighbors, Sharpeville and Boipatong.

The U.N. Security Council, reacting to the upsurge of violence in South Africa, Tuesday condemned apartheid and demanded its "immediate eradication." The United States abstained in the vote; the 14 other members supported the resolution.

In Washington, however, the State Department issued a statement saying the United States "deeply" regrets the raids on the townships. "These repressive measures are bound to obscure and put into question the (South African government's) professed intentions in dealing with the problems of the country by reform and concession," the State Department said.

The raid began at 2 a.m. in Sebokeng. The soldiers, who made up the bulk of the task force, formed a tight cordon around the sleeping township, taking up positions 10 yards apart with automatic rifles at the ready. They were backed up by scores of heavy armored troop carriers.

A battery of searchlights was switched on to illuminate the dim township, and police officers dressed in camouflage uniforms began the house-to-house searches.

After each house was searched, a red label was fixed to the door and to the clothing of each member of the household. A police spokesman, Col. Leon Mellett, explained that this was to show that they had been "vetted" and could move about freely.

Before dawn, as people began leaving for work, they were stopped at roadblocks and searched, after which each was painted on the hand with a red dye before being allowed to continue. When they returned from work this evening they had to show either their red labels or stained hands before they were allowed back into the township.

The labels bore a printed slogan that said, "I am your friend, trust me," part of a public relations attempt to present the raid as an operation to rid the township of troublemakers.

Lantern Makhaye, 18, said he was "real scared" when five police officers pounded on the door of his parents' house at 3 a.m.

"They came into my room and shone torches in my face," Makhaye said. "They opened drawers and rummaged about among my clothes, but they didn't say anything, and they didn't take anything. After a few minutes they left and went next door."

"The searches were fairly superficial," a second police spokesman, Lt. Henry Beck, said. "Most lasted only a few minutes, but at some houses, when we found the sort of things we were looking for, they took longer." Beck said these "things" included illegal firearms.

Mellett said none of those arrested had been charged under the security laws. He said most had been charged for such things as violations of the pass laws, which apply only to blacks, and for possessing marijuana, banned pornographic literature or stolen goods.

In the late afternoon, the raiding force pulled out of Sebokeng and began similar searches in Sharpeville and Boipatong.

A third police spokesman, Lt. Johan Barnard, said these other raids had not been planned. "We decided to go there because we had the manpower available in the area," he said.

Trevor Manuel, a spokesman for the United Democratic Front, said the variety of violations for which people had been arrested showed that "the authorities are looking for something they cannot find under beds or wardrobes. The anger of the people over rentals and lack of participation in government does not hide in those places." Ishmael Mkhabela, a spokesman for the National Forum, said the operation was a "declaration of war."

The white liberal Progressive Federal Party also criticized the raids. A spokesman, Philip Myburgh, warned that the use of national servicemen in a police role would increase resistance to conscription.