Two powerful bomb explosions rocked downtown Johannesburg's busiest streets today, injuring 18 persons, two of them seriously, in the biggest guerrilla attacks so far in South Africa's premier city.

In a continued clampdown on reporting of the country's civil strife, the Pretoria government expelled a third foreign correspondent in six days today, ordering a reporter for a major Israeli newspaper to leave by Saturday.

One of the bombs today exploded in a crowded Wimpy fast-food restaurant during the lunch hour, wrecking the facility and wounding 16 whites. A two-month-old baby was among those injured.

About 20 minutes later, another bomb exploded a few blocks away outside the entrance to a popular hotel owned by the Holiday Inn chain. It injured two black men but did little damage to the hotel.

There were two other guerrilla attacks today, on an empty sports stadium in Johannesburg's black township of Soweto and a shopping complex in the northern tribal "homeland" of KwaNdebele.

The incidents appear to be an attempt by the black underground to launch a counteroffensive against the South African government's emergency declaration and crackdown on black activist organizations.

There have been nine guerrilla attacks, six of them in major cities, during the past 10 days. South Africa has said that the outlawed African National Congress is responsible, but there has been no acknowledgment of the attacks from the organization's exile headquarters in Lusaka, Zambia.

One black person was injured by gunfire at the shopping complex. The motive for the attack was not immediately apparent.

The government's Bureau for Information, the only authorized source of information during the nearly two-week-old emergency, called the two bomb attacks "callous acts of terrorism."

The bomb that exploded in the Wimpy restaurant was placed at the foot of a stairway in the center of the room, which was crowded with lunch-hour customers. The explosion demolished the room, mangled furniture and blew out glass at the front of the restaurant. Witnesses said it was remarkable that no one was killed.

The baby, Glenda Geduld, was hit on the head by a piece of flying glass as her mother, Michelle Geduld, walked past the restaurant.

Today's bombings followed a massive car-bomb explosion outside a Durban beachfront hotel on June 14 that killed three young white women dining in a restaurant there and injured 68 other persons, most of them white.

If, as many observers here suspect, the incidents are the work of ANC guerrillas, it suggests a change of strategy by the group to attack "soft" targets with the deliberate intention of causing civilian casualties.

Previously, the ANC has said that it avoided such targets and directed its attacks primarily at military installations and government buildings.

Where civilians have been hit -- such as a car-bomb that killed 20 persons and injured 200 outside a Defense Force building in Pretoria in 1984, and a limpet mine that killed six and injured at least 61 Christmas shoppers near Durban last December -- the organization has said either that the civilians were inadvertently hit in an attack aimed at military personnel, or that underground agents had launched unauthorized attacks.

At a conference in Zambia last year, the ANC leadership came under strong pressure from young activists who had slipped out of South Africa to approve attacks on civilian targets. The leaders resisted the pressure. Eventually a compromise was reached in which it was agreed that less care would be taken to prevent civilian casualties during attacks on institutional targets.

Congress leaders continued to assert publicly that the organization was opposed to attacking civilians, but since then the civilian casualty toll has risen steadily.

With Pretoria's June 12 declaration of a state of emergency, followed by massive detentions of political activists, the internal pressure may now have overwhelmed this thin line of restraint within the ANC.

Such a shift could damage the organization's hopes for greater diplomatic recognition in the West. Today's attacks occurred as ANC president Oliver Tambo met in London for the first time with a senior Foreign Office minister

The Israeli reporter, Dan Sagir of the influential Tel Aviv daily Haaretz, said he had been given no reason for the expulsion order but believed it was because of a critical report on the situation here similar to one that appeared in last week's Newsweek magazine.

Newsweek's correspondent, Richard Manning, was ordered yesterday to leave the country. A Dutch citizen working as a CBS cameraman, Wim de Vos, was expelled last week.

Sagir, who has been in South Africa for two years, said that although his article was critical of the administration, he had not deliberately violated stringent new regulations governing press coverage. He said, however, that the rules were so broadly phrased that "whatever you do, they can kick out any foreign journalist if they want to."

He said he believed the government was particularly sensitive about what he had written because there was a rising tide of anti-South African feeling in Israel, which Pretoria feared was damaging its close relationship with that country.