JOHANNESBURG, South Africa (AP) _ Three black constables who shot to death four blacks and wounded four others were detained Tuesday for an investigation into whether the shootings were justified, police headquarters reported.

In a separate incident, police said black attackers hacked and bludgeoned to death six blacks and then set their bodies afire, apparently to avenge the stabbing of a man outside a bar in the Ciskei homeland.

The government reported the number of violent incidents declined 70 percent in the second half of 1986 following the imposition of a nationwide state of emergency in June. The report also said the number of deaths related to the violence had dropped from 665 in the first half of the year to 251 in the second six months.

The three unidentified constables under investigation were suspended from duty and taken into custody in the shootings Monday night in the Tantje township near Grahamstown in Cape province, a police statement said.

The three detained officining. Opposition groups criticized the short training period when the government announced plans in September to hire 6,000 special constables to support the regular police on township duty.

According to the police statement, the three constables were guarding a school when ''they were shot at from a certain direction, whereupon they answered the fire and four people were killed and four wounded.''

''Police are urgently investigating the justifiability of the actions of the special constables,'' the statement added.

Dave de la Harpe, a Grahamstown civil rights lawyer, said the victims were shot inside three houses across the street from the school. He identified the slain victims as Lele Mbenge, an elderly man; a woman, Topsy Peter, and a couple, Johnson and Jane Mantile, who had three children.

The force of special constables was created to augment the 48,000-man police force, which is about 40 percent black, in black areas.

More than 2,400 people have been killed violent incidents over the past 2 1/2 years, according to unofficial estimates.

The special constables at first received three-week training courses, but that was doubled after an outcry from opposition groups. A senior police spokesman said Tuesday he did not know the current length of training or how many of the special constables were on active duty.

Black policemen have been constant targets in South Africa's unrest. More than 50 black policemen have been killed, and more than 600 homes of black officers have been destroyed, usually in fire-bomb attacks.

The Ciskei incident occurred Sunday night in Mdantsane, a black township near the port city of East London. A Ciskei police spokesman confirmed six men were killed and their bodies burned, but declined further comment.

South African Press Association said the killings apparently were meant to avenge the fatal stabbing the previous night of Mkhuseli Jaambi, 22, outside a bar in Mdantsane. The report said one of the intended victims escaped and informed police, who found the burned bodies of the victims, all aged between 18 and 22, at a scrapyard.

Ciskei is one of four black homelands declared independent by South Africa, so political violence there is not included in the daily South African police ''unrest reports.'' None of the homelands is recognized abroad.

The government says killings of blacks by other blacks have accounted for most of the violent deaths during the state of emergency.

Tightened press restrictions imposed in December bar reporting of ''subversive statements'' on a range of issues as well as on security force actions, unless authorized by censors.

In its report, the government's Bureau for Information said the drop in violent incidents showed the state of emergency had ''saved lives and protected property.''

It said security-force action resulted in 34 percent of the deaths in the first half of the 1986 and 30 percent for the July-December period, while the percentage of killings attributed to ''black agitators'' rose from about 65 to 70 percent in the second half of the year.