Four Americans were killed and nine persons, including five Americans, were injured today in an explosion apparently caused by a bomb aboard a TWA Boeing 727 airliner en route from Rome to Athens.

TWA officials said the blast took place at 2:32 p.m. local time (6:32 a.m. EST), southwest of Athens, as the plane was descending at 15,000 feet prior to landing. The plane, with a 3-by-4-foot hole in its right side just in front of the wing, landed normally at 2:40 p.m.

Officials said they had received no threats against TWA Flight 840, which originated yesterday morning in Los Angeles. One anonymous telephone call in Beirut today claimed that a bomb had been placed aboard the aircraft by a pro-Libyan Palestinian group called the Ezzedine Kassam unit of the Arab Revolutionary Cells in retaliation for "U.S. imperialism . . . including its attack against Libya" last week. State Department officials said the group was unknown to them.

Coming nine days after the U.S.-Libyan confrontation in the Gulf of Sidra, the apparent bombing raised questions of possible retaliation by Libyan leader Muammar Qaddafi or Islamic extremist groups he is said to support. It follows a series of terrorist attacks beginning with the hijacking last summer of a TWA jet to Beirut, where its U.S. passengers were held hostage for 17 days.

But Qaddafi denied any role in today's explosion. "This is an act of terrorism against a civilian target, and I am totally against this. I reject this," he said in an interview with United Press International in Tripoli.

The four dead, including three Annapolis residents -- a Greek-born widow, her daughter and 8-month-old granddaughter -- were sucked from the aircraft through the hole caused by the explosion. Their bodies were found by shepherds six miles outside the town of Argos. Details on Page A32.

TWA officials and Greek police identified the dead man as Alberto Ospina of Stratford, Conn., a Colombian-born American. The victims from Annapolis were identified as Demetra Stylianopoulos, 52, her daughter, Maria Klug, 24, and infant granddaughter Demetra Klug. They were en route to the port city of Piraeus, from which the family had moved to Annapolis in 1971, to take care of family business following the death of Stylianopoulos' husband, Andreas, in November.

Charles Adams, vice president for TWA's international division who flew here tonight from London to head the company's investigation, said he could not be absolutely certain yet that a bomb caused the explosion. But he made clear that this was the firm belief of both the company and Greek authorities.

Flight 840 leaves daily from Los Angeles by Boeing 747. The flight lands in New York before taking off again for Rome. In Rome, the 747 is replaced by the smaller 727 for the last portion of the flight to Athens and Cairo.

Most of the passengers on the flight, were Americans, TWA officials said. A number got off the plane in Rome, and nine new passengers boarded.

For the Rome-Athens leg, the plane was carrying 114 passengers, plus one infant and seven crew members. Top capacity for the 727 is 140 passengers.

Adams said the plane had been "sealed" by Greek officials pending the arrival here Thursday of an eight-man team from the FBI and additional investigators from the Federal Aviation Administration and Italian security agencies.

Adams said TWA officials here were not allowed aboard the plane today. But, he said, initial indications were that the explosion took place in the passenger compartment, rather than in the baggage hold.

The hole in the side of the aircraft is just below the window at row 10. Ospina, Adams said, had the row 10 window seat, while the other three dead were believed to be in row 11.

Officials agreed that the rest of the passengers were spared because the explosion took place at a relatively low altitude, where outside pressure was low at approximately four pounds per square inch. The low pressure, TWA international spokesman Stephen Hechscher said, also enabled passengers to be held in by their seat belts, which most of them presumably had fastened for landing.

The explosion damaged none of the plane's major operational systems, enabling pilot Richard F. Petersen, of Sarasota, Fla., to bring it to a relatively uneventful landing.

The aircraft was sequestered in a remote part of Athens International Airport tonight.

Adams said tonight that he did not believe TWA had become a specific target for such attacks, but noted that both it and Pan American fly numerous routes in the eastern Mediterranean. "I don't think we're being picked on," Adams said. "I think it's just the toss of the cards."

On Dec. 27, simultaneous terrorist attacks were launched in the departure areas of international airports in Rome and Vienna, leaving 20 dead. The United States charged Libyan involvement in those assaults, for which Palestinians believed to have links to the Abu Nidal terrorist organization asserted responsibility.

On Feb. 4, a Libyan executive jet en route from Tripoli to Damascus, Syria, was forced by Israeli fighter aircraft to land in northern Israel. Israeli officials did not find the Palestinian militants they had believed to be aboard and allowed the plane to continue to Damascus. It was rumored that Abu Nidal was one of those aboard the Libyan plane.

After that incident, Qaddafi vowed to take revenge at some point in the future against U.S. aircraft anywhere in the world and warned passengers not to fly on commercial American airlines.

Qaddafi renewed his threat last week after a clash with U.S. naval forces in the Gulf of Sidra off the Libyan coast. U.S. Navy aircraft and warships, retaliating after being fired at by Libyan SA5 missiles, attacked both the missile site and several Libyan coastal patrol boats.

The group that asserted responsibility for today's explosion has claimed other attacks. It is named after Ezzedine Kassam, a Palestinian guerrilla who fought the British occupation of Palestine in the 1930s and who was killed by the British.

In the interview with UPI, Qaddafi asked, "How could we do this? I don't know who could do such a thing. The passengers on a plane like this would be mixed, not just Americans. Anyone could be on that plane."

Qaddafi said if the Reagan administration blames Libya for the attack and uses the incident as an excuse to send the U.S. 6th Fleet back to the Libyan coast, "that would mean America did this act to create a justification for attacking our country."

Last summer's TWA hijacking and the December attacks in Rome and Vienna led to stepped-up security precautions in most international airports and by leading commercial airlines. TWA vice president Adams said tonight that all of those procedures had been followed when passsengers on flight 840 boarded the aircraft in Rome.

Adams said that, following last week's Libyan-U.S. clashes, TWA general managers in Rome and Athens were stationed at airports for all incoming and outgoing flights. In addition, he said, TWA's international security director was in Rome this morning, conducting an audit of compliance with company procedures. "He did not check this flight, however," Adams said.

Adams, who spoke to The Washington Post after meeting here with TWA local officials and Greek security officials, said the company was confident of its baggage security procedures, which he said included electronic screening and a matching of all checked luggage with passengers before it was loaded on the aircraft.

Although he declined to detail actual security procedures, Adams said that "once a bag is put on the aircraft we know that the passenger who turned in that bag is on board." If not, he said, the bag is taken off.

Baggage that was taken aboard the flight in Los Angeles and New York was placed in containers that were sealed and transferred to the 727 in Rome without rechecking.

Looking weary and frustrated, Adams asked, "How the hell could somebody get something on? We have a pretty good system."

Although they originally thought that the explosion had taken place in the baggage hold, TWA officials said they now are reasonably sure that it was in the passenger cabin. Rome airport officials said that all passengers and their hand baggage had been checked and X-rayed on boarding there.

U.S. officials in Washington speculated that plastic explosives, which would not have appeared in X-rays, could have been brought aboard the plane in hand baggage, or that a bomb could have been placed aboard by maintenance personnel.

A statement issued by the Greek government said: "We condemn the barbarous terrorist act aboard the TWA aircraft and repeat that terrorism undermines peace and democracy."

Italy's defense minister, Giovanni Spadolini, said military actions are "never effective" in dealing with terrorist attacks and cautioned the United States against making any military response to the bombing.