Convoys of heavily armed police and troops backed by helicopters raided at least seven neighborhoods here today, arrested dozens of persons and threatened violent repression of any demonstration against the government of Gen. Augusto Pinochet.

In the widest ranging military operation since Pinochet declared a state of siege Nov. 6, districts of low-income housing around Santiago were invaded almost simultaneously this morning by armored cars, trucks and buses carrying police and troops. Some raids reportedly continued into the afternoon.

In some instances, the government forces patrolled streets and checked cars and identity cards for up to several hours without making arrests or searching homes. In other districts, homes were searched and hundreds of persons were detained temporarily by security forces, who sometimes randomly seized people found on the streets.

The object of the raids appeared to be to intimidate residents with a show of force only three days before planned nationwide antigovernment demonstrations. "They said they were going to be tough," said the Rev. Enrique Gonzales, a priest in the north Santiago district of Colon Oriente. "They said they would burn down the church."

One of the largest security operations occurred in Santa Julia, a working-class neighborhood in the southeastern area of the city. Residents there said troops and police moved through the streets for two hours as officers randomly detained hundreds of men.

Witnesses said about 200 men were gathered on a vacant field for an identity check, while several hundred more were held in other areas. However, the Rev. Patricio Rojas, a local priest, said only seven persons finally were taken away when the troops left the area.

Another government force returned to the shantytown of La Victoria in southwest Santiago only nine days after a massive military roundup there. The Rev. Pierre Dubois said in a telephone interview that four truckloads of troops and buses of police patrolled the area for more than an hour, but made no arrests. "I think it was meant as a mass psychological intimidation," he said.

Government authorities had offered no information by late today on the raids, and Chilean media had not been allowed to report on them. Officials at the Chilean Human Rights Commission said that partial reports indicated that at least 50 persons had been jailed in addition to those temporarily detained.

The display of force followed other government moves during the past several weeks meant to silence the political opposition, drive leftist political parties underground and remove criminals and activists from the most restless urban neighborhoods.

Troops and police made house-to-house searches in two poor Santiago neighborhoods this month and arrested 600 persons. Yesterday, military forces carried out a third such mass roundup in the coastal city of Vina del Mar, arresting 191, according to official reports.

Hundreds of others have been arrested in their homes or offices and interrogated in barracks or secret police installations. There have been isolated reports of torture, although none of deaths. Human rights groups say more than 500 persons have been sentenced to three-month periods of internal exile in remote regions of Chile, including more than 400 with criminal records.

Government officials invariably have described their measures as meant to uproot organizations of leftist terrorists. However, human rights groups say the principal victims of the crackdown have been the opposition press, middle-level activists in political parties and residents of poor neighborhoods.

Opposition leaders, who have described next week's national protest as the most important confrontation yet in Chile's 18-month-old political crisis, acknowledged that the military offensive has hampered severely efforts to organize the two days of demonstrations.

The entire leadership and many grass-roots activists of the Communist-backed Democratic Popular Movement have gone underground to avoid arrest, depriving many working-class areas of their normal protest organizations. Centrist political parties, meanwhile, concede that they have had difficulty communicating with their own followers because of media censorship and bans on public meetings.

The sole Democratic Popular Movement leader still available for public comment, Communist spokesman Patricio Hales, said today that most leftist coordinators in poor urban neighborhoods have been withdrawn by their parties and relocated in "backup neighborhoods," where security forces are considered unlikely to conduct searches.

Many key activists thus have been able to avoid arrest but have been neutralized politically, Hales said. "We cannot afford to risk everything now," he said.