It was a normal Saturday morning scene as shoppers milled through the streets of Salisbury, hurrying to finish errands before the noon closing hour. Suddenly, 10 police vans swopped in on Gordon Avenue, sealing off the street, closing traffic, ordering stores to close their doors.

Rifle-toting police converged on startled shoppers. "There's no need to panic," an officer shouted through a public address system from the top of a van. "This is a routine police check. Please cooperate and move to the center of the road for a search."

The incident was the latest step in the new "beat the bomber" campaign, part of the massive new security program launched in the capital this month after the first major incidents of urban terrorism.

Two bomb blasts on successive Saturdays - one a 60-pound explosive planted in Woolworths that killed 11 and injured 76 and the other, just 300 yards from the prime minister's office, that caused no casualities - have had an immense impact on sleepy Salisbury. Although Rhodesia has been fighting a guerrilla war for almost five years, the cities have until now been safe from assault.

Now shoppers and businessmen are nervous about walking downtown, limiting their movements to essentials duties. Whites suspiciously eye Africans carrying parcels or bags. People stick to the streets rather than risk the many arcades, considered dangerous for all their nooks and crannies.

The sporadic police cordon-and-search operations downtown reflect the jitteriness of both officials and civilians. Members of the bomb squad wade through the crowd searching shoppers, checking their parcels. All cars, in transit or parked, are examined.

No one moves, including customers trapped in stores, until the operation is over.

Officials say the searches are necessary for psychological as well as practical reasons, to let potential bombers know police are on alert and to reassure whites that everything possible is being done to prevent another blast.

White morale is crucial. Already nervous because of the increasingly tenuous political situation, more than a thousand whites are leaving Rhodesia each month. The government is aware that the new dimension of urban terrorism is likely to lead to higher emigration figures.

The terrorism in the capital has made whites recognize their vulnerability - political and physical - more than ever. Further incidents could be the last straw for many of the 270,000 whites who dominate the breakaway colony's 6 million blacks.

Africans have borne the brunt of the anti-bomb campaign. Roadblocks have been set up outside Salisbury's African townships where police check all blacks and their vehicles for suspicious materials. Officials believe the bomber is a black nationalist belonging to the Zimbabwe African National Liberation Army, a guerrilla unit based in neighboring Mozambique, Zimbabwe is the African name for Rhodesia.

Several Africans have complained bitterly about the recent police raids in the black suburbs, which are aimed at flushing out possible guerrillas. Last week a squad hit Kambuzuma township, surrounding houses and firing "warning shots" that, one African said, went through his windows. One of the residents of the area is Albert Mugabe, brother of guerrilla leader Robert Mugabe.

Security guards now man the entrances to every store, hotel and office building in Salisbury around the clock. No one gets in until packages, purses and briefcases are searched, Suspicious-looking visitors are sometimes frisked.

Several offices have run bomb scare drills. Southhampton House holds the record, emptying 11 stores in seven minutes.

The ministry of Information has launched a publicity campaign, issuing a collection of large signs for business use outlining the specifics on bombs and where they might be concealed.

The tension in the capital was reflected in one incident this week. A maid found a black box in a hotel room ticking slowly. Police immediately swarmed to the room, only to find a British cameraman's lighting equipment being recharged. They did not leave until his suitcases and drawers had been fully searched.