A remote-controlled car bomb today shattered and set aflame the Palestine Research Center, the last remaining political office of the PLO here. The bomb also caused severe damage to a nearby building that housed the news agency and temporary embassy of the Libyan government.

At least 18 people were killed and more than 100 injured in an explosion so powerful that it hurled one car off the street into the entrance of the Libyan offices and shook buildings and rattled windows throughout the central Hamra shopping district of predominantly Moslem West Beirut.

Thick brown smoke enveloped the area as flames darted from both buildings. The blast lofted books and papers above the research center and ripped away entire walls. Cars burned on the street. The screams of victims could be heard all around--those who were trapped in the offices and apartments in both buildings and those who came stumbling outside with faces bloodied or disfigured.

"I was on the first floor when the whole world went black around me," said a Palestinian cleaning woman at the research center. "I ran to the elevator but the falling ceiling blocked the way, so I raced to the balcony and jumped."

Although Red Cross ambulances and Lebanese civil defense volunteers rushed immediately to the scene, it took fire trucks about 25 minutes to arrive.

In the meantime, passers-by shinnied up gutter pipes to rescue the trapped and stopped cars and taxis to take casualties to hospitals.

As a swelling crowd of journalists, French troops of the multinational peace-keeping force, Lebanese Army soldiers and the agonized friends and relatives of those inside gathered around the burning buildings, the scene bordered on pandemonium. The relatives scuffled with the French soldiers and Lebanese Army soldiers argued with firemen when they finally arrived.

The explosion was the most violent incident of the last 10 days in Beirut, which also has seen five bombings in western sector and two artillery shellings of the Christian east.

This renewed cycle of violence comes after four months of relative calm.

As is often the case here, authorities gave no indication that they had any idea of who might have been responsible for the attack today.

A shadowy group calling itself the Front for the Liberation of Lebanon from Foreigners claimed credit, as it has in the other bombings, but few here seemed to know whether the organization really exists.

The ultra-right Voice of Free Lebanon, a radio station founded by the Lebanese Forces Christian militia, blamed Syria.

Shafiq Hout, the PLO's diplomatic representative in Lebanon, described the explosion as a "cold-blooded Israeli crime." The PLO's Cyprus-based news agency Wafa said it was part of an Israeli campaign to "rid Lebanon of any Palestinian presence" through an ongoing "terror campaign."

The explosion here came eight days after a car-bomb blast at a three-story office building used by Palestinian troops and Syrian soldiers in the eastern Bekaa Valley town of Shtawrah.

The bombing today is certain to complicate the negotiations for the withdrawal of Israeli, Syrian and Palestinian troops from Lebanon.

Sources close to the PLO indicated here tonight that the attack would more than likely cause them to insist on extremely strict guarantees for the security of Palestinian civilians as the price of withdrawal of the soldiers.

The research center was established here in 1965 and had existed primarily as an information center and the repository of Palestinian archives until last September, when the elaborate state-within-a-state that the PLO had erected here was dismantled and the center became the sole remaining political office in Lebanon.

It had been the target of two car-bomb attacks last July and August, which caused moderate damage. When Israeli soldiers entered West Beirut in September they ransacked the research center and took away about two-thirds of its library.

But officials at the center had managed to reconstruct much of the collection by photocopying archives in other libraries here and by soliciting collections from private owners here. They had been planning to announce by late April that the library had been substantially rebuilt.

The research center was allowed to remain here as part of the deal worked out by special U.S. Middle East envoy Philip C. Habib last September to get the PLO to agree to evacuate its fighters. Hout's diplomatic mission was also allowed to remain, but he used offices at the research center because his own offices here are occupied by Lebanese Army soldiers. Three Lebanese Army soldiers posted outside the research center today were among those killed. An estimated 17 plainclothesmen around the center were wounded in the blast.

The explosion occurred just minutes before 2 o'clock on what had been a calm afternoon. The indications were that it came from a remote-controlled bomb in a car parked at the front entrance ot the research center. Police here said tonight that the blast was caused by 130 pounds of a special high explosive. They said the force of the blast was equivalent to more than 300 pounds of TNT.

Hanneh Jiyris, the wife of the center's director, who had stopped by on a visit at what was the traditional close of the Saturday workday here, was among those killed in the explosion. Her husband Sabri was not seriously injured. The dead included three people trapped in an elevator, a concierge and a telephone operator.

Officials at the nearby American University of Beirut Hospital, where most of the casualties were taken, said tonight that it had received 115 victims from the blast, six of whom were dead on arrival. A hospital spokesman said another six were taken immediately to surgery and 10 others were admitted to the hospital with serious fractures, cuts and burns.