President Ferdinand Marcos reiterated today that he will reinstate armed forces chief of staff Gen. Fabian Ver if he is acquitted in a murder trial, but he also promised a top-to-bottom reorganization of the country's military.

Marcos made the promise during a campaign appearance in this central Philippines city, considered an opposition stronghold.

The president called the election in the face of criticism of his administration at home and abroad for its handling of a Communist insurgency, reported abuses of human rights and an economic crisis.

He told reporters today that he is willing to postpone the election from Jan. 17 to Feb. 7, in a concession to the opposition, which had requested a longer campaign.

Marcos also announced in his campaign speech that the International Monetary Fund has agreed to release the delayed third installment of a $615 million loan. The official Philippine News Agency said the third installment was $110 million.

In Washington, an IMF spokesman said agreement had been reached by a working group and was awaiting approval by top fund officials.

Ver, now on leave, is expected to become an issue in the campaign. He recently was tried for covering up a military plot to kill opposition leader Benigno Aquino, and is expected to be acquitted when the court announces its verdict Wednesday.

The position of the acting chief of staff, Lt. Gen. Fidel Ramos, and the duration of Ver's return will be decided by senior officers, Marcos said. He gave no details of the promised reorganization.

Marcos stressed that the changes will extend from the general staff down to the lower ranks.

The United States, the Philippines' principal ally, has made it clear that it opposes Ver's return even if he is acquitted. But Marcos has said that he is committed to the reinstatement, although he made no public mention of how long he will keep Ver on the job.

Defense Minister Juan Ponce Enrile warned in Manila of the political repercusssions for the election if Ver returns to the military, saying many Filipinos consider Ver guilty.

Addressing a luncheon for businessmen, Enrile said: "Realistically speaking, if he is returned to the service, whether there is an election or not, you can, I think, anticipate some adverse reactions from some sectors of our body politic."

Meanwhile, the Philippines' leading churchman, Cardinal Jaime Sin, asked all contending political parties, the commission on elections and the armed forces to ensure free and orderly elections.

Speaking at a meeting of church, military and political leaders, Sin said the vote comes at a time of political crisis, economic distress and escalating violence.

He said it is essential that the elections commission, which has been accused of partiality to the Marcos government, be seen as credible.

Marcos, when asked if he will retire the more than 30 generals whose terms had been extended, to give way to younger officers, said the reorganization will depend on the needs of the Philippines' counterinsurgency campaign.

"We will decide and determine what is needed for the country's anti-insurgency," he said. "We will seek advice of all concerned."

He said he was in favor of promoting younger officers, but stressed that some generals of retirement age are needed in some key areas in which no replacements could be found.

The United States has been critical of both the lack of military reforms and Marcos' handling of the insurgency waged by the Communist New People's Army, which is estimated by the Pentagon to have 16,500 armed regulars.

The Reagan administration has said the spread of the insurgency threatens the two U.S. military bases in the Philippines.

Marcos denied that he felt any pressure from the strong statements from Washington officials during the past few weeks asking for reforms in his government.

He told a Cebu businessmen's convention earlier, however, that one of the reasons he was calling the election was because of "a systematic effort at home and abroad to downgrade" his government.

Thousands of flag-waving schoolchildren lined the streets to cheer Marcos here.

He was accompanied by his wife, Imelda, whom he has often called his "secret weapon" in election campaigns.

She demonstrated her campaign skill, singing local songs to 2,000 persons gathered in a stadium.