Heavily armed gunmen, carrying out what appeared to be a carefully planned operation in Moslem west Beirut today, kidnaped an American priest who heads the U.S.-based Catholic Relief Services office here.

The kidnaping came only hours after the body of a French military observer was found near the Museum Crossing of the line separating west Beirut and Christian east Beirut.

There was no indication that the two incidents were related and by late tonight no group had claimed responsibility for either attack. But there has been a surge of kidnapings, robberies and car thefts in the Moslem sector of the wartorn Lebanese capital in recent days. The increase in lawlessness and general breakdown in authority has resulted in an exodus of Americans and foreigners from west Beirut and the departure of many from Lebanon.

The Rev. Lawrence Martin Jenco, 50, of Joliet, Ill., was seized by at least seven gunmen after two cars intercepted his chauffeur-driven Pontiac just behind a police station on Sadat Street, a few hundred yards from his apartment.

In New York, a spokeswoman for Catholic Relief Services said Jenco had been en route to medical treatment for a severe heart condition. "Father Jenco's physicians are extremely anxious that Father Jenco be released for continued treatment at the earliest possible moment," she said.

U.S. Ambassador Reginald Bartholomew and the Rev. Luciano Angeloni, the Vatican's envoy to Lebanon, met with Shiite Moslem militia leader Nabih Berri to enlist his efforts in seeking Jenco's release.

Berri, who is also justice minister, helped negotiate the release yesterday of a kidnaped Swiss diplomat. He was quoted by state-run Beirut radio tonight as saying that "condemning abductions is no longer enough," but he gave no details of further action.

Khaled Kronfol, Jenco's driver, told reporters that when the two cars blocked him, "I saw gunmen in the first car, pointing guns at us. Then men in the second car fired eight or nine shots in the air." Three men got into Jenco's car, he said. "They put us in the back seat. One drove, one put a gun to my head and a third stuck a gun against Jenco's chest."

Witnesses on the street said they saw about seven or eight gunmen. I saw gunmen dressed as militiamen in one of the cars shoot at traffic behind them and speed away.

At a sea-front corniche, Kronfol said, Jenco was forced into one of the gunmen's cars and Kronfol was hit on the head with a gun and forced into the trunk of Jenco's car. Kronfol, who suffered a head injury, escaped and alerted the Catholic Relief Services office of the abduction.

Jenco, a member of the Roman Catholic Servite Order, has headed the Catholic Relief Services program in Lebanon since last October. He served in Thailand before coming to Beirut, The Associated Press reported.

In Joliet, his brother, John F. Jenco, said the priest knew "it wasn't an easy assignment. He always said that if he were to die, he'd like to die as a missionary. I hope that isn't the truth, but if that's the way it's to be, I guess God knows best."

Jenco is the second American cleric to be kidnaped in west Beirut during the past year and the fifth American to be seized by gunmen in 11 months. None has been found.

The Rev. Benjamin Weir, a Presbyterian minister, was kidnaped on his way to work in May. Before that, Jeremy Levin, a Cable News Network correspondent, was kidnaped March 7 and U.S. Embassy political officer William Buckley was abducted March 16. Peter Kilburn, a librarian at the American University in Beirut, has been missing since Dec. 3.

Yesterday, two Lebanese priests were briefly detained and their car stolen. Today, state-run Beirut television showed bomb damage done to two churches in the mainly Moslem northern port of Tripoli.

The main two Moslem militias in control of west Beirut are the Druze Progressive Socialist Party, headed by Walid Jumblatt, who also is public works and tourism minister, and Berri's Amal movement.

A spokeswoman for the Catholic Relief Services here said the charitable organization has provided millions of dollars in aid and reconstruction projects in Lebanon, including rebuilding of hospitals.

"We received no threats because we work with every group," she said. "Everybody is our friend. We are not a political organization. Although we are a Catholic organization, we are giving to every sect."

The seizure of Jenco followed the discovery of the body of Lt. Col. Claude Cueno, 45, deputy commander of the French military observer force here, who was found shot dead with one bullet wound in his head.