Twenty persons were killed and scores injured today when soldiers opened fire on antigovernment demonstrators in the key sugar-growing province of Negros Occidental in the central Philippines, regional and provincial officials said.

The demonstrators were among thousands who went on strike on the eve of the 13th anniversary of the declaration of martial law by President Ferdinand Marcos.

The Philippine armed forces have been placed on alert in anticipation of widespread protests planned Saturday by moderate and left-wing groups throughout the country.

Provincial police commander Col. Arnulfo Obillos said today that soldiers began shooting when a gun was fired from among approximately 2,000 demonstrating farmers, transport drivers and unemployed sugar workers during a demonstration in Escalante, a coastal town about 60 miles north of the Negros Occidental capital of Bacolod city.

Officials said 20 protesters were killed in the shooting that took place after soldiers used tear gas and water jets to try to disperse the crowd. Today's toll of violence was the worst in two years of nationwide protests spurred by the August 1983 assassination of opposition leader Benigno Aquino Jr.

The demonstrators had barricaded the streets of Escalante yesterday at the start of a three-day general strike called in several towns of the province, where an insurgency by Communist rebels has grown faster than anywhere else in the Philippines as a result of the collapse of the sugar industry, according to local officials and plantation owners.

In the central Philippine city of Cebu, the Philippines' second-largest city, police charged into a barricade on the outskirts of the city and arrested 167 strikers, 10 of whom were injured. Riot police in full battle gear patrolled the streets and guarded buildings.

A strike by transport workers left Bacolod, about 300 miles south of Manila, deserted and major entry points to the city were blocked by timber and other obstacles. Offices, stores and schools had been closed since yesterday.

In Manila, students marched through the downtown area, and a clash was reported at Adamson University as boycotting students tried to enter the campus.

Marcos declared martial law on Sept. 21, 1972. Even though martial law was lifted in 1981, government critics maintain that Marcos retains sweeping powers.

The strike in Negros Occidental was spearheaded by the left-wing Bayan group, which includes trade unions, students and the militant clergy. It has the support of the sugar workers, many of whom have lost their jobs in the current slump in the sugar industry, which has been hit by declining world market prices.

Since last October, more than 400,000 sugar workers have lost their jobs on Negros Occidental, officials said.

The province, which accounts for about 55 percent of the country's total sugar production, is described by Philippine officials as the next powder keg in the growing Communist insurgency that is being fueled by the spread of hunger and malnutrition among displaced sugar workers and their families.

Philippine officials now say that the insurgency, with a local force of New People's Army guerrillas about 1,000-strong, is growing faster in Negros Occidental than in Mindanao, the guerrillas' stronghold.

In a recent interview, provincial police constabulary chief Obillos said, "People are suffering. When people are hungry, they are vulnerable."

About 100 children die each month from malnutrition, according to church and international relief officials. Six percent of the preschool children in Bacolod have third-degree malnutrition, meaning that their body weight is 65 percent of normal, according to church officials, and malnutrition affects about 20,000 persons in the province, they said.

Rice costs about 3.4 pesos a pound, about half of a small family's daily need. The minimum legal wage for a farm worker is 32 pesos a day, about $1.72, but no one is getting it now. The average pay is about 50 percent to 60 percent of the minimum, and some workers receive as little as 10 pesos a day, when there is work, according to local businessmen.

In March, guerrillas raided the Visayan Maritime Academy in downtown Bacolod city, taking more than 400 weapons. In May, guerrillas raided the 3rd Scout ranger company in Isabela town, killing 10 soldiers.

On March 29, on the anniversary of the founding of the New People's Army, the military wing of the Communist Party of the Philippines, red graffiti glorifying the guerrillas was scrawled on walls and road signs all over the province.

"These two incidents indicate that the NPA can move into the city and countryside so easily only with the support and sympathy of the populace," said Fred Pfleider, a sugar planter who is also the president of the Negros Occidental chapter of Bayan, in a recent interview.

Local military officials said the government was losing the psychological battle with the rebels.

Col. Isagani de los Santos, head of the Negros tactical command post in charge of counterinsurgency in the province, said, "We want to gain lost credibility."