An American and three Filipino opponents of President Ferdinand Marcos were sentenced to six years imprisonment and fined to six years imprisonment and fined $1,333 each today after they admitted plotting to kill Marcos and take over the government.

The American, August ,McCormick Lehman, 26, of Nashville, Tenn., admitted his participation in the plot although he had previously told Amnesty International that the government had no case against him.

There had been reports during his 4 1/2 years in prison indicating that he had been tortured but special correspondent Benard Wideman reported that he looked healthy and robust in court today, showing no ill effects from the imprisonment.

Lehman's treatment since his arrest the day after Marcos declared martial law on Sept. 22, 1972, has been unusual for a person accused of so serious a crime.

While he was reportedly kept in a dungeon at first, he was later moved to comfortable quarters and allowed to marry the daughter of an officer in Marcos' personal security forces. The wedding was held at the presidential palace, Marcos' security chief gave the bride away and the president himself congratulated the groom. Lehman and his wife and baby have been living in a detention house in the officers' section of one of Manila's military camps.

The defendants tried today are among 10 persons, including two other American and prominent Filipino political figures, who have been charged with at least seven attempts on Marcos' life and with trying to raise an army of 4,300 men to stage a coup.

The two other Americans and a Briton are being tried in absentia, although the Briton is believed to be dead.

Lorry Tractman, described as California businessman who was to supply the rebels with equipment, and Robert Pincus of Jericho, N.Y., are believed to be living in the United States. The Briton involved in the case, Brian Borthwick, is believed to have died recently in a Singapore prison.

The others sentenced today along with Lehman were Eduardo Figueras, son of a former mayor of Manila, and businessmen Manuel Crisologo and Antonio Arevalo.

"Now that I have had time to reflect on this attempt to overthrow the government, the least that I can say is that I was certainly wrong," Lehman told the seven-officer tribunal.

"By admitting my guilt, by admitting my participation in this affair, I accept your judgement and the punishment you deem necessary."

In late 1975 Lehman told Amnesty International that Philippine authority agreed that they had no charges against him and that he was being held as a material witness.

The sentences imposed today will be reviewed automatically by the chief of staff, of the armed forces, the Secretary of defense and Marcos.

Defense attorneys said they expected the sentences to be reduced and one of the lawyers noted that the men had spent more than four years in jail that could have credited to their sentences. With time off for good behavior "they'll be out in a few months anyway," he said.

Not before the court today were former newspaper publisher Eugenio Lopez Jr., former senator and presidential candidate Sergio Osmena Jr., the alleged mastermind of the plot, and Osmena's son, Sergio Osmena III.

Former senator Osmena is in exile in the United States and is being tried in absentia. He has called the government charges a "amlicious canard." His son and Lopez had their trials postphoned until March 24 because the Supreme Court is considering a petition from them asking that a charge of attempted murder be dropped. They are expected to plead "not guilty."

Following the declaration of martial law, Marcus' brother-in-law took over the Lopez newspaper and one of the presidential advisers took over a Lopez-owned radio-television network.

The prosecution claimed the plot was devised shortly before Marcos suspended the Philippine congress and declared martial law. The defendants were taken into custody in 1972 but formal charges were not filed until last year.

Figuresas submitted an affidavit to the court claiming that the group also planned to set fire to government buildings throughout Manila but the martial law decree upset their timetable.

He said the elder Osmena offered $333 for every building burned but the would-be arsonists were arrested before they could carry out their orders.