Another campaign organizer for opposition presidential candidate Corazon Aquino was reported killed today in election-related violence that many observers here are calling the dirtiest and bloodiest of any political contest in this country's memory.

The killings reported today by Aquino campaign headquarters, which include a youth believed to be the campaign organizer's son, bring to at least 28 the number of persons killed since Feb. 6 who are connected to the opposition or a citizens' poll-watching group called Namfrel. In addition, Aquino headquarters cited 30 cases in which opposition or Namfrel workers were beaten, wounded or shot at and nine cases in which such persons were arrested or missing.

To a greater extent than in the past elections, it seems, the killings have continued beyond last Friday's actual polling day. Opposition spokesmen say this continuing violence may be in part intended to intimidate Aquino supporters and discourage participation in a wave of demonstrations being planned to back her claim to have won the election. The opposition expects Marcos to be proclaimed the winner in the National Assembly, which he controls, on the basis of a canvassing of electoral returns that the opposition charges are fraudulent.

President Reagan has suggested that both sides may be guilty of election fraud, which he said has also been known to occur in the United States. But the issue of election violence has had far less of an impact in official Washington.

Election campaigns in the Philippines long have been practically synonymous with violence, as well as voting fraud and other "irregularities." While no Philippine president ever has been assassinated, hundreds of lower level officials and campaign workers are periodically killed or wounded in election violence that is regarded as almost a routine part of political life.

Aquino headquarters reported today that Arsenio Cainglet, a 43-year-old campaign organizer in the town of Moncada in Aquino's home province of Tarlac, and a youth believed to be his son were slain. It said Cainglet was shot to death by four armed men near his home and that his son disappeared after he heard gunshots and ran to the scene. Later, a body was found in the vicinity, but it could not be identified positively as that of the son because of numerous wounds on the victim's face, Aquino headquarters said.

It said Cainglet had been warned by a military or police officer before the elections to stop campaigning for the opposition.

So far, according to military figures, at least 95 persons have been killed in election-related violence in the past two months, 36 of them on election day, last Friday, and the weekend immediately afterward. The military says 30 of the victims belonged to Marcos' ruling party and 16 to the opposition, while the rest had no established political affiliation. However, military figures include persons killed by Communist guerrillas in ambushes that may not be strictly related to the election.

When an official U.S. observer team led by Sen. Richard G. Lugar (R-Ind.) visited the Philippines during the election, several members, including Lugar, were exposed to the results of violence.

According to journalists accompanying a team led by Lugar to the violence-stricken province of Tarlac, the delegation appeared reluctant to hear the story of an Aquino supporter who was smashed in the face with an M16 rifle butt by a member of a paramilitary force supporting Marcos.

However, the group expressed great interest in the plights of registered voters who showed up at the polls only to find that their names were not on the voters' list -- a politician's nightmare to which they could relate more easily.

The most prominent victim so far has been Evelio Javier, the former governor of Antique Province in the central Philippines. Javier was gunned down Tuesday in the provincial capital by armed men reputedly linked to his political archrival, Arturo Pacificador, a Marcos loyalist who serves as assistant majority floor leader in the National Assembly. Pacificador, who was accused by prosecutors of involvement in the ambush slayings of seven Javier followers on the eve of a parliamentary election two years ago, has denied involvement in Javier's death.

Javier's bullet-riddled body was flown today to Manila, where about 1,000 persons accompanied his coffin from the airport to a requiem mass at a Roman Catholic church. Aquino, the widow of assassinated opposition leader Benigno Aquino told several thousand people attending the mass that she empathized with Javier's family and that he had died for his country. She said Javier has shown that "there will be more sacrifices for all of us."

Most of the victims of political and election violence are unsung campaign workers and low-ranking party officials. Some of the killing may be to make good on the threats and coercion that mark Philippine elections. Another reason, according to Philippine politicians and military sources, is that political rivalries at the local level tend to become intensely personal.

An incident yesterday in the National Assembly spoke volumes about the attitudes here toward electoral violence. As opposition and progovernment legislators scrutinized the official vote tallies, an opposition member noted that the signature of the opposition's official representative in the province of Quirino did not appear on the official vote tally document as required. The reason, he said in a matter-of-fact tone, was that the representative, Francisco Laurella, had been kidnaped the day after the election and his bullet-riddled, mutilated body recovered from a river a few days later.

Assembly speaker Nicanor Yniguez angrily banged his gavel for silence. The account of Laurella's death had "no bearing" on the proceedings, said Yniguez, who was Marcos' campaign manager. "It is enough to show that he has not signed the certificate of canvass."