Two terrorist bombs exploded here yesterday, critically injuring two persons, as police and security forces intensified a special alert on the eve of the Camp David summit conference.

Authorities said they feared the Palestine Liberation Organization, which claimed responsibility for yesterday's blasts, would attempt to disrupt the Camp David talks with more bombings here. Police and army units stepped up patrols and intensified roadblocks in Arab districts.

The last fatal terrorist explosion in Israel, which killed one person and injured 46, occurred Aug. 3 in Tel Aviv's central market.

The most seriously injured in yesterday's explosions was a police bomb disposal specialist who immigrated to Israel in 1973 from the United States. The name of the victim, a Vietnam veteran and former Los Angeles policeman, was withheld pending notification of relatives. He reportedly lost both legs and an arm in the blast.

Officials said the bomb went off as the disposal specialist approached it after police received a report of a suspicious-looking object in a storage depot of propane gas cylinders in the German quarter here.

The other bomb exploded underneath an empty tourist bus parked near the usually crowded Jaffa Gate to the Old City. There were no casualties.

The bus had been transporting members of the International Federation of Travel Jounalists.

Witnesses to the first blast said the bomb had been planted between a parked truck and a fence. The explosion, which could be heard thorughout Jerusalem, sent shards of metal and glass flying over a wide area.

The storage depot houses hundreds of propane cooking gas cylinders. Authorities said the terrorists may have hoped to set off a chain reaction of explosions, but the cylinders did not explode.

The injured police specialist had dealt with more than 25 bombs - including two that exploded while he was attempting to defuse them. He had developed a reputation as one of Israel's best bomb experts. When the two other bombs went off, he was wearing protective equipment and was ont injured.

In an interview published in Friday's editions of the Jerusalem Post, the demolition expert talked about the hazards of his work in Israel, where explosives have been found daily for several weeks. He was identified in the interview only by his first name, Stephen.

"Of course I'm scared. If I wasn't, I'd be a nut," he said in the interview. "If your name is on a bomb, that's it. And you have to believe in something apart from yourself, although I'm not religious in the strict sense."

He said he was drawn to bomb work by what "you could call a certain kind of Zionism."

As part of the terrorist alert, plice and security forces have increased deployment in public places, particularly traditional terrorist targets such as bus depots and markets. A special security alert also has been issued to Israeli embassies and private firms aboard, as well as Jewish organizations with close ties to Israel.

Following the Aug. 20 terrorist attack on El Al airlines drew members in London, Isreaeli securith agencies began preparations that have included roadblocks, mock evacuations, the use of night flares in strategic areas and special briefings for those involved in supervisiong crowded public places.

Israel's state-owned television has carried a short film showing a woman asking other bus passengers if they own a package stored in rack, then alerting the bus driver to call police.

Partly as a result of the campaign, the number of suspecious objects redorted has inceased dramatically. In Jerusalem one Friday last month, police received 141 reports of ossible bombs. One turned out tobe an explosive.

The increased public awareness has saved lives, police say. They cite one case last month in which the driver of a truck that had just pulled into a gasoline stoage depot noticed a device planted under his vemicle. He called police and a bomb was removed, averting a possible conflagration.

Police and the Isreaeli Army in the occupied territories also have stepped up security investigations in recent weeks, making numerous arrests and beaking up what are termed terrorist cells. In some of the raids, nquantieis of explosives have been confiscated.