LOMBA RIVER, Angola (AP) _ Dug into trenches and trading cannon bombardments with the enemy, guerrillas unaccustomed to conventional warfare claim they are holding firm against an offensive by government forces.

More accustomed to hit-and-run tactics during a 15-year war, rebels of the National Union for the Total Independence of Angola are battling to protect the administrative and farming center of Mavinga, which they captured in 1981.

During the week, Soviet-made jets dropped bombs near villages and in cornfields around the district, which is home to 20,000 people.

Mavinga is about 135 miles northwest of Jamba, headquarters of the guerrilla group known by its Portuguese abbreviation, UNITA. The guerrillas have been fighting for a share of power since Marxists took control of the government following independence from Portugal in 1975.

UNITA officers say the government launched the offensive last month to embarrass guerrilla leader Jonas Savimbi, on his first visit to Portugal since independence. Portuguese officials have agreed to meet Savimbi, although they recognize the Angolan government.

''They wanted to sabotage the trip of our president, to say, 'You're in Portugal, but we're in Mavinga,''' said Gen. Arlindo Pena Ben-Ben, UNITA's military chief of staff.

Savimbi postponed his trip, originally set for Jan. 12, to lead 12,000 guerrillas against the offensive. At least 9,000 government troops accompanied by tanks had driven through 75 miles of guerrilla-held territory.

''At that time, we were not in control of the situation; now we are,'' said Ben-Ben.

He said the forces of the government - known as the Popular Movement for the Liberation of Angola - were running low on supplies, harassed by guerrillas attacking the convoys, and unable to make accurate bombing runs because of the danger of guerrilla anti-aircraft missiles supplied by the United States.

The jets make up to six flights across the river each day at altitudes of 20,000 feet, dropping two bombs at a time.

Craters from phosphorous and shrapnel bombs within a few hundred yards of villages indicate the targets of recent bombing runs.

A woman who was seven months pregnant lost her baby and suffered severe cuts when a phosphorous bomb landed outside her house, and several women were hurt by shrapnel when a bomb landed in a cornfield, said Lt. Col. Cristiano Sussula.

''They were aiming at the villages but flew away because of our anti- aircraft defense. They had to drop the bombs abruptly,'' said Sussula.

The guerrillas say the planes also drop chemical bombs, which send up a grayish-green smoke. Villagers attracted to the smoke on Jan. 17 collapsed, coughing, vomiting and losing control of their bowels when they approached, said doctors at Mavinga.

Three died in two days, and others were transferred to Jamba for more sophisticated treatment, doctors said.

Journalists taken to a site near Mavinga saw a circle of dead bushes, with leaves turned yellow on the stem. A week after the bombing, a strong metallic taste lingered in the mouths of those who went close enough to inhale the air around the site.

The pounding from multiple-firing cannons, the thuds of mortar shells and the rumble of tanks reverberate day and night through the bush.

Rainy-season offensives are rare in Angola. Ben-Ben said the government launched this one in desperation, seeking a position of strength because it is under pressure from other African countries to negotiate a settlement.

Savimbi and Angolan President Eduardo dos Santos agreed to a truce in June, but it collapsed within weeks.

As of Jan. 10, the guerrillas say they had 46 dead and 149 wounded, and had killed 207 government soldiers, wounded 175 and injured four Soviet advisers. There are no firm figures on civilian casualties, but UNITA reported at least 17 deaths and dozens of injuries from bombs.

Ben-Ben said Cubans are also participating in the battle, in small numbers, but he can't prove it unless he captures one. A Cuban presence in southern Angola would violate a December 1988 agreement with South Africa that calls for all foreign troops to withdraw from Angola and for South Africa to grant independence to neighboring Namibia.

Last week, Cuba suspended the withdrawal of its remaining 19,000 troops out of anger over a guerilla attack that killed four of them. UNITA said it did not know Cubans would be stationed at the water purification plant that was attacked.

UNITA was not a party to the 1988 accord, but Savimbi said last year his soldiers wouldn't attack retreating Cubans, only those who continued fighting.