A lone black flag stood today at the Shatila refugee camp here, lost in the weeds and garbage of the field where more than 300 Palestinians and Lebanese were buried in a mass grave a year ago after Christian Phalangist militiamen went on a rampage.

No memorial service was held or is planned to mark the first anniversary of this grim episode in the blood-soaked history of this hapless land. But memories of that three-day spree of murder that began on the night of Sept. 16, 1982, still remain starkly fixed in the minds of Shatila residents.

Indeed, fits of panic that the Phalangists are about to return still grip the camp from time to time, the latest on Aug. 28.

"We are afraid the Phalangists are going to come back," said Zuhair Mohammed, 23, a grocery store owner on one of the main alleys where the butchery took place. The alley's walls still bear the pockmarks of bullet holes.

The Phalangist threat has become a specter haunting Sabra and Shatila, the two adjacent camps on Beirut's southern outskirts where the massacre took place.

Zakiya Hamad, the 33-year-old mother of eight children, said she told her crying daughter the other night, "Be quiet or the Phalangists will hear and come get you."

The child stopped crying.

Ironically, the same Phalangist militia today is calling for an international committee to investigate the killing of Christians by Druze militiamen in a score of villages caught in the mountain fighting between the two groups over the past two weeks.

To this day, nobody really knows how many died in that terrible slaughter of men, women and children during those three days a year ago. The International Committee of the Red Cross stopped counting after it buried 328 persons, most of them in a mass grave at the southern entrance to this camp.

The Lebanese government has estimated the toll at 460, but the Israeli commission that investigated the Israeli role in allowing the Phalangist militia to enter the camps put the deaths at between 700 and 800.

Here in the camp, however, the figure most often used is around 1,000. Another 1,000 residents are still said to be missing, some of them reportedly being held in the Israelis' Ansar prison camp in southern Lebanon.

Never well publicized in the international outcry over the massacre of Palestinian refugees was the fact that perhaps up to half of those killed or still missing were Lebanese civilians, many of them Shiite Moslems, who lived on the edge of Shatila where the worst butchery took place.

The population of the two camps today--now estimated at 15,000 to 20,000--has dwindled considerably. This is partly because the Lebanese government has not allowed homes destroyed outside the legal limits of the camps to be rebuilt and partly because those Palestinians not part of the 1948 flood of refugees into Lebanon have had to leave the country.

School enrollment is down by 10 percent, according to the United Nations Relief and Works Agency, and Italian sources from the peace-keeping force involved in protecting the camp say 30 to 40 percent of the old population has departed.

Residents of the two camps say the reason there is no memorial service to mark the day of the massacre is to avoid angering the Lebanese government in the already tense situation.

"We can do nothing," one resident said. "We cannot even raise a black flag. Kuwait declared a memorial while we cannot."

For the remaining Palestinians and Lebanese living in and around the two slum camps, the quality of life--never very good--appears to have deteriorated considerably since the departure of the Palestinian guerrillas from here a year ago.

The streets are littered with debris and garbage, and children play cheerfully in the mud. The stench of uncollected and rotting garbage around the bustling central market of Sabra is almost unbearable. There is still no water in much of the two camps. Health conditions are worse and electricity even scarcer than in the rest of the city these days.

Dr. Amir Hamawi, who works in one of the camp hospitals, said sanitary conditions were "absolutely worse" than a year ago. He noted that the number of cases of typhoid, or suspected typhoid, this summer rose to 20 to 30 a week.

"This summer there were more cases than in any other year," he remarked.

The only collection of garbage visible during a visit to the camps today was being conducted by a private Lebanese company owned by the millionaire contractor Rafiq Hariri, who is taking part in the current negotiations for a cease-fire and a political settlement to the fighting here.

The one significant improvement here has been in security, thanks mainly to the French and Italian peace-keeping troops who have fixed positions throughout the two camps and run patrols through the main streets at night.

"Their presence is very necessary," Dr. Hamawi said. "They have given the people a sense of security."

Mohammed, the grocery store owner on Pine Street, the alley where so many--including 10 in his own family--were killed, agreed, but said the residents wanted more Italian and French posts inside the camp.

The young men of the camps are still in the habit of sleeping elsewhere at night when the tension rises in the city. They do so for fear that the Lebanese Army or security forces will come on sweeps to arrest them as they did almost daily last fall.

"This is not a safe area," said one of Mohammed's neighbors gathered at his grocery store. "The Americans must come and help us," another said.

Residents were relieved to see that neither the French nor the Italians fled when the two camps were shelled Sept. 6 and three Italian and four French soldiers were wounded. Officers of the two forces say privately that some of the shells were 155mm ones coming from Phalangists' guns in east Beirut, apparently aimed at sowing panic among the Palestinians.

If so, the fears of camp residents about the Phalangists coming back in one way or another may not be all that imaginary.

"I am not afraid of the shells because it is up to God whether I am hit," said one in the group around Mohammed's store. "But if the Phalangists come again there is nothing I or God can do. They will kill us."