Communist guerrillas are stepping up their challenge to the government of Philippine President Ferdinand Marcos, attacking targets in greater numbers than ever before and increasing their recruitment, according to western diplomats and Philippine sources.

The campaign by the New People's Army (NPA), the armed wing of the Communist Party of the Philippines, is forcing the government to devote more resources to its counterinsurgency effort at a time of severe economic straits.

In a meeting with military leaders Monday, Marcos ordered the deployment of more troops and helicopters in the southern Philippines, where Communist guerrillas killed 24 people in three attacks over the weekend, news agencies reported. The Philippine leader called for an "intensified peace and order campaign" in northern and eastern Mindanao, the island that has become the scene of some of the heaviest rebel activity.

While the New People's Army still is considered a long way from its goal of toppling the government, analysts see the organization as steadily gaining ground. Lately the guerrillas' recruiting efforts have been taking advantage of hard economic times in the Philippines, where more than half the population lives below the poverty line.

The result, according to former Philippine president Diosdado Macapagal, is that the country--the most important U.S. ally in Southeast Asia--is drifting the way of Nicaragua, whose U.S.-backed authoritarian government was overthrown by leftist rebels.

"What happened in Nicaragua is exactly what is starting to happen here," Macapagal, now an adviser to the moderate anti-Marcos opposition, said in an interview. He said that to forestall the Communists' gains, the United States should pressure Marcos to hold a fair election. After lifting eight years of martial law in 1981, Marcos easily won reelection to a six-year presidential term in a poll boycotted by most opposition groups on the grounds that the contest was manipulated.

Macapagal blamed the 1972 declaration of martial law and what he called the Marcos "dictatorship" for much of the Communists' success.

"Ironically, the martial law government was imposed to save the country from 1,500 Communist rebels," who he said were originally confined to Isabela province on the island of Luzon. "But after a decade they're much stronger. Now they're all over the country. It's precisely this regime that has brought danger to the country." The former president, who was succeeded by Marcos in 1965, said the New People's Army now counts 6,000 to 10,000 guerrillas.

A Defense Ministry spokesman put the group's strength at 4,000 to 6,000 fighters, but insisted that only about 2,500 were armed and that they relied on a "mass base" of only 50,000 supporters and sympathizers. However, a well-informed western diplomat quoted a provincial governor as telling him privately that there are at least 12,000 New People's Army guerrillas in the country, about half of them in Mindanao.

"Up to last month the NPA had not really done much," this diplomat said, but in January an unusual number of incidents were reported--at least 28 guerrilla attacks--with some of the rebel units consisting of up to 200 fighters instead of 10 or 12 as before.

"The NPA has been recruiting heavily, trying to get more arms and demonstrate its strength," the diplomat said. He said the escalated rebel activity threatens to wipe out government gains in Mindanao against Moslem insurgents of the Moro National Liberation Front, whose strength has steadily declined since the early 1970s.

In response to the Communists, the Philippine Army last month moved three battalions out of the Moslem area of West Mindanao to fight the rebels in the northern and eastern parts of the island, he said.

The diplomat stressed, however, that while the New People's Army can make life difficult for the government, it cannot take power without significant external support.

According to another western diplomat, the New People's Army guerrillas "lately have improved their capability and willingness to involve themselves in larger scale operations." He cited an incident a month ago in which a rebel force of about 200 took over the coastal town of Mabini in the Mindanao Province of Davao del Norte for a day before retreating back to their inland mountain strongholds.

The guerrillas captured the municipal hall and local police station without a fight, seizing weapons, ammunition and other supplies and holding a flag-raising ceremony in the town. A firefight ensued when the rebels unsuccessfully tried to take over the local police constabulary headquarters, resulting in one casualty on each side.

In other incidents last month, two Philippine Army battalion commanders and a Mindanao mayor were among those killed.

In an effort to reassure Filipinos about the rash of attacks by Communist rebels so far this year, Marcos insisted that the incidents "do not prove that they have become stronger and pose a serious threat to national security."

He told reporters Jan. 27 that the rebels were merely trying out new tactics in using up to company-size forces in their attacks.

"But ultimately they get pushed into a corner where they have to make a last-ditch stand," Marcos said. "We have always felt that when they start moving in bigger groups, that is the end of their guerrilla group. When you hear of some 50 or 100 men going together, there is something wrong."

Since the president spoke, however, a number of successful new attacks have been reported.

In one of the latest incidents, a New People's Army force variously estimated to number between 80 and 200 guerrillas raided government installations near the town of Tagum in Davao del Norte Friday, killing 18 persons including 10 civilians, Manila newspapers reported. Scores of others were reported wounded. Listed among the dead were a policeman and six paramilitary troopers.

In a separate attack, about 50 insurgents led by a woman raided a logging firm last week and disarmed a squad of police constabulary troops guarding it, the Army's Southern Command reported.

In the town of Gandoni near the central Philippines city of Bacolod, about 100 suspected Communist guerrillas wearing Philippine Army uniforms attacked a government office, killed two paramilitary troopers and seized arms and ammunition, military reports said.