JOHANNESBURG, NOV. 18 -- South African Army troops and security police moved into the black township of Soweto before dawn today to back up actions by the town council to break a 17-month-long rent boycott.

There were no reports of violence as armored vehicles patrolled streets in the township on the outskirts of Johannesburg. Council officials warned residents to pay their rent arrears or face forcible eviction as early as Friday.

A police spokesman said the troops were in Soweto in a "protective capacity."

The rent strike has been the most sustained demonstration of civil disobedience here in 35 years. While it has spread to other segregated townships throughout South Africa, Soweto is the principal arena of protest.

Some residents reported that they had been told they could pay half their back rent and avoid forcible eviction. The last time large-scale evictions were carried out, in October 1986, it led to violence that claimed 34 lives.

The rent boycott, launched to protest the June 12, 1986, imposition of a national state of emergency, has cost Pretoria an estimated $200 million in lost revenue, and caused a budget deficit in Soweto last year of more than $55 million.

About half of the township's 110,000 households have refused to pay rent and service charges at various times, but the council has evicted few of them, apparently out of fear that wholesale removals could ignite popular resistance.

There are an estimated 2.5 million inhabitants crowded into 38 square miles of South West Township, Soweto's full name. The Soweto Civic Association has said the boycott will continue until the township council, which is backed by the all-white government in Pretoria, resigns and rents are lowered.

Rents for two-room, matchbox-like brick houses average about $20 monthly, plus an additional $30 for utilities. The unemployment rate in Soweto is estimated to be 52 percent, and many who work earn the equivalent of about $100 a month. In August, Soweto Mayor Nelson Botile said that rent defaulters would no longer be evicted, a policy change designed to promote the government's planned sale of houses to residents. Two-room houses are expected to be sold for approximately $335, including the land.

The black Soweto town council, elected in 1983 in a widely boycotted vote, has tried various measures to break the strike, such as threatening to evict its leaders, including Albertina Sisulu, a president of the national antiapartheid coalition, the United Democratic Front.

Sisulu said officials with lists approached selected houses beginning at 4:30 a.m. today and, while armored personnel carriers filled with white soldiers waited in the street, warned residents that they would be evicted Friday if they failed to pay their back rent.