South Africa police arrested more than 600 blacks, including 100 children, today, in a new crackdown on black dissidents and announced seizure powers unusued since World War II to counter the U.N. arms embargo.

In one black township, police opened fire with shotguns on a group of blacks who threw rocks at police cars.

Police spokesman Maj. Gen. David Kirel said about 50 black youths degan stoning police vehicles in Cardock, a sheep farming town about 450 miles south of Johannesburg, forcing police to open fire. He said there were no injuries.

Police Brig Henry Abbot, describing the early morning police raids on a black township near Pretoria, said most of those arested were black "agitators."

Abbot said more than 200 of the blacks arrested were taken in for "pass law" offenses. South African law requires every adult black to carry the bulky "pass" at all times. It shows his thubprints, and contains information on his origin and tribal area and on where he is permitted to work.

Abbot said about 100 childred aslo were detained. These mostly were "inneed of care," and would be investigated individually to determine whether they should be committed to institutions, he said.

For months students have been roaming the streets of South Africa's black townships and boycotting classrooms to protest the country's segregated education system.

The South African government said recently that it had reached agreement with leaders of most of the country's black homelands to phase it with an identification system administered by the homelands. The new system has not yet been implemented.

Simultaneously with the arrests, Economic Affairs Minister Chris Heunis announced drastic measures - last enforced during World War II - giving the government powers to procure and seize goods from private citizens without compensation.

The measures, which become operative by proclamation today, were in response to last week's mandatory arms embargo against South Africa imposed by the U.N. Security Council and supported by the United States.

The new measures include powers to "seize, without legal process, the goods in question, should anybody wilfully refuse or fail to comply with orders" issued by the economics minister.

Under the measures Heunis also may "order persons capable of supplying goods and services, of manufacturing, producing, processing or treating any goods, to supply, deliver or sell such goods to the minister." Thus the South African government could force foreign companies to produce weapons.

South Africa's original clampdown on black activities was announced by Kruger Oct. 19 - the closure of two black newspapers, the arrest of their editor Percy Qoboza and 47 other black activists and a ban on 18 black groups. That action prompted the U.S. arms embargo.

Meanwhile, Finance Minister Owen Horwood announced today that South Africa will increase construction of low-cost housing for nonwhites over the next three years.

An extra $287 million will be spent on housing for blacks, "colored (people of mixed race) and Asian Moves are also in hand to introduce home ownership plan for city-dwelling blacks, Horwood said.

The housing program is to be part of a package of measures aimed at gentle and selective stimulation of the economy, which has been in the doldurms for more than two years.