Trials of several hundred suspects in an alleged military plot to overthrow the Iranian government will begin Thursday, President Abol Hassan Bani-Sadr announced today, after a meeting with Ayatollah Ruhollah Khomeini.

Ayatollah Mohammad Behesti, head of the majority Islamic Republican Party and Supreme Court chairman, described the process as a forthcoming "execution of a group of conspirators" and said the proceedings would be televised.

He also attacked the National Front -- a coalition of nationalist forces, and one of the few tolerated political organizations -- saying that "some of its members have been arrested."

The Moslem fundamentalist leader did not call for the banning of the National Front, but said he was determined to crush all opposition, adding: "The revolution has always tried to preserve democratic liberties but this does not mean we shall remain inactive in the face of plots and crimes."

Other Iranian officials said a Catholic missionary school with about 800 students in Tehran was closed because its priests allegedly spied for Israel. sAccording to Tehran radio, the Catholic teachers denied spying but were ordered not to leave the country.

The official Iranian news agency Pars quoted the officials as saying all "missionary" schools would be closed as a result of the incident.

Iran's oil installations were under heavy guard against saboteurs as a result of the 48-hour security quarantine called because of the alleged plot.

The arrested leaders have admitted they planned to bomb vital refineries and pipelines, the state radio said, warning the danger still existed.

"There is a big possibility that the remnants of the ominous triangular plot of America, Israel and Iraq are still at large and will threaten the oil industry," the radio said.

The country's borders and air space, closed early today are due to reopen early Thursday morning.