A powerful bomb ripped through the headquarters of Zimbabwe's ruling party here this afternoon, killing six persons and injuring between 120 and 150.

A government spokesman said no leading members of Prime Minister Robert Mugabe's Zimbabwe African National Union were among the dead. The government news agency Ziana reported, however, that a woman senator, Sunny Takawira, was injured. Most of the injured were released after treatment, but 20 were hospitalized.

The blast, which shattered store windows for blocks around the party headquarters, came during the lunch hour when the streets were full of Christmas shoppers. Many party workers were believed to be at lunch and thus escaped injury. Political commissar Mayor Urimbo, the number four official in the party, was on the ground floor but was unhurt.

The government broadcasting network quoted unofficial sources as saying a white South African had been arrested, but a police spokesman categorically denied the claim, saying, "There is nothing to the report." He said police had no immediate information on who planted the bomb.

The claim, however, could well be the first in a round of charges that the South African government was involved in the explosion. Mugabe's government recently has stepped up claims that the white-minority Pretoria leadership is trying to destabilize this country.

Mugabe has charged that some whites in Zimbabwe, including members of former prime minister Ian Smith's Republican Front, are trying to overthrow his government, but he has also included defeated black political opponents in those accusations. Nine whites, including a member of Parliament, have been arrested in recent weeks under emergency laws, but no charges have been brought yet.

In a speech last week, Mugabe accused some whites of having "one foot in Zimbabwe and another in South Africa." He called upon them to be loyal to the country or leave.

There is a possibility, however, that dissidents in Mugabe's party, which has known many rifts, could be responsible for the incident. Security at the party headquarters always has been tight, with guards using metal detectors on visitors, which would have made it difficult to sneak the estimated 65 pounds of explosives into the building.

Tensions between blacks and whites in the country have grown since the advent of black rule last year, particularly in recent months. Whites are leaving at a rate of about 50 a day, reducing the white population to about 180,000, less than 3 percent of the 7 million total.

There were some antiwhite incidents at the bombing site today. At least two whites, one a woman, reportedly were beaten.

The explosion was heard all over downtown Salisbury and brought police cars, fire trucks and ambulances to the crowded downtown area within minutes.

All those killed were at a next-door bakery, which was crushed under the weight of the collapsed roof and wall of the four-story party headquarters. The government spokesman said the bomb was placed in the top-floor board room. The explosion blew off half of the roof.

Rescue workers carried bloody, sometimes screaming, victims from the two buildings.

Many of the injuries were caused by flying glass. Almost all the windows in a department store across from the party headquarters were blown out, leaving Christmas tinsel blowing in the wind.

The only major bombing in Salisbury during the seven-year guerrilla war occured in 1977, when a blast in a Woolworth store killed 11.