Rebel forces have extended a campaign of economic sabotage and intimidation into the region around Angola's capital, shutting off electrical power in this city of 1 million inhabitants for the past two days.

The destruction Monday night of an unspecified number of transmission towers and high-tension electricity lines near the town of Dondo, about 100 miles southeast of Luanda, was the second successful attack on a vital economic target in recent days by guerrillas belonging to the National Union for the Total Independence of Angola (UNITA).

The guerrillas planted bombs in the main rail yard in the port of Benguela two weeks ago, damaging or destroying a dozen locomotives. UNITA's repeated attacks along the important railway linking Benguela to the eastern part of the country already had put key sections of the line out of operation during the past three years.

The recent attacks mark a new level of economic warfare in the eight-year-old effort by the UNITA guerrillas, who have received support from South Africa and sympathy from political leaders in the Reagan administration, to dislodge the Soviet-backed Angolan government.

Government officials said that they expected electricity to be restored to Luanda by the end of the week and dismissed the mining of the remote, unguarded transmission towers as an isolated act of terrorism by "criminals trained by South Africa." Luanda depends for its electricity on the power station at Dondo and on a hydroelectric dam at the nearby village of Cambambe.

But the complete blackout of the city for more than 48 hours appeared to have had a demoralizing effect on many citizens, causing tensions to rise. Residents were digging in for the possibility of a lengthier disruption of daily life than the government estimates would suggest.

Long lines quickly formed at gasoline stations as word of the attack spread Tuesday morning. Food, already extremely scarce in the war-scarred capital, is being stockpiled urgently. Water is being stored in any available container, especially by residents of the city's many high-rise buildings, which depend on electric pumps to carry water to the upper floors.

Luanda is a city already crippled economically and physically by the abrupt collapse of Portuguese colonial rule in 1974 and the war among contending African movements that followed.

Cuban troops helped the Popular Movement for the Liberation of Angola (MPLA) defeat two other African groups and secure international recognition as the legitimate government in 1975. UNITA, with extensive South African logistical help, regrouped after its defeat and is now conducting an increasingly effective economic war against the Luanda government.

UNITA's attacks have taken a dramatic turn in the past six months toward sabotage, apparently aimed at frightening away the growing number of western technicians and businessmen helping Angola in its efforts to restore the economy. Diplomats here believe that the campaign also may be aimed at derailing U.S.-sponsored efforts to negotiate agreements with Angola and South Africa that would end the insurgencies in Angola and Namibia and require the MPLA to get the Cubans to leave.

These diplomatic efforts appear to have made progress since the beginning of the year. In that same period, UNITA has concentrated on economic targets, progressively moving toward Luanda.

One week after South Africa and Angola agreed to a disengagement plan in February, UNITA assaulted an important diamond mine in Angola's sparsely populated far northeastern region. An effort to reopen the mine in August drew a new attack, and it has been shut down again.

The bloodiest act of sabotage appears to have been the explosion by UNITA of a large car bomb in April near a building housing Cubans in the city of Huambo. UNITA claimed to have killed a large number of Cuban soldiers, while the government and foreign sources here report that 78 persons were killed, including a number of Angolan civilians.

Despite a heavy Soviet Bloc presence here, Angola's Marxist government depends on western investment and expertise for its foreign exchange earnings of $2 billion a year, which come almost entirely from petroleum sales.

UNITA turned its attention to the Gulf Oil installation in the Cabinda enclave in September, planting a bomb under an onshore pipeline. Damage was quickly repaired and little oil was lost, company officials said, but two Angolans who lived near the site died in the blast.

The government has been ambivalent about publicizing the economic sabotage campaign, generally announcing only enough details to dispel rumors or deflate overstatements in UNITA's accounts of the attacks.

There has been no official announcement of the bombing of the locomotives in Benguela, although the attack quickly became known from accounts by visitors and by workers at the rail yard. The power blackout initially was ascribed to technical problems before being announced as a sabotage action.

Until now, foreign residents of Luanda do not appear to have been scared off by UNITA's campaign, and there has been no popular unrest in the capital, which has shrugged off 24-hour power failures due to technical problems in the recent past.

But in the current atmosphere of uncertainty, the strike against the Luanda power supply has set nerves on edge. It has underlined a vulnerability that already was apparent in a city that on most days has little visible security beyond a few soldiers stationed in front of some government offices.

"These groups have been able to reach greater areas of the country" recently, the chief of staff of Angola's armed forces, Col. Antonio Franca Ndalu, acknowledged in an interview several days before the blackout occurred. "They're not attacking our soldiers or trying to take a city, but they can put bombs in factories and kill civilians. It is impossible for us to be everywhere at once."