As the Sri Lankan government today began evacuating by cargo ship the thousands of minority Tamils trying to flee communal rioting here, the overflowing refugee camps were jittery with talk about fears of attacks by Sinhalese, the island nation's dominant ethnic group.

"We are sitting ducks in here," said a 32-year-old Tamil clerk housed with 4,000 other refugees in a compound on the outskirts of Colombo. "They can throw bombs in here and massacre us all like they did in Lebanon."

Despite official assurances that the Tamil refugees were in no danger, at least 33 people were killed in violence in the capital today, government spokesman Douglas Liyanage said. The deaths brought the official total in a week of communal rioting throughout the country to more than 120, but unofficial reports say the number is considerably higher.

Those killed today included 15 looters shot by troops and 15 Tamils killed by Sinhalese mobs, officials said. At least 400 people were reported arrested in Colombo today.

Twelve suspected Tamil terrorists were arrested in Colombo's central business district and Liyanage told reporters that police believed Tamils were trying to infiltrate the capital for a major terrorist action.

Authorities also said nine Tamils were attacked and burned to death by a Sinhalese mob yesterday after police stopped a man carrying a handgun and detonators for a bomb. Police said a crowd gathered around the suspect, and that when 10 other Tamils ran away, they chased them, beat them and set them afire while they were still alive. Police said nine of the Tamils died of burns and one survived.

While casualty and arrest figures from the rest of the country are spotty, a day-long drive through the interior today made clear that the systematic burning and looting of Tamil-owned businesses and homes by Sinhalese mobs has been widespread in rural Sri Lanka. Virtually every town and village showed evidence of arson attacks on Tamil properties.

Despite the Tamil refugees' fears of attack, security appeared spotty at the Pillaiar Hindu Temple compound adjacent to the Hindu College on the outskirts of Colombo yesterday. About a dozen government troops patrolled inside the camp but only one was stationed at the main entrance. Although visitors are prohibited, I and another western journalist spent more than an hour inside talking with some of the 4,000 refugees before we were spotted by guards and escorted out.

While Colombo's overwhelming majority of Sinhalese have made no menacing moves toward the 35,000 Tamil refugees packed into 10 hastily erected camps, the Tamils said ethnic hatred was running so high in the city that they are afraid that the thinly spread government security forces would be unable to protect them from an attack if it should occur.

Government officials insisted that the refugees were safe in the camps and said that all Tamils who ask to be transported to Jaffna, 200 miles away in the predominantly Tamil peninsula at the northern tip of Sri Lanka, will be sent there as soon as possible.

The first group, from a camp at Ratamalana Airport in south Colombo, left this morning for a seven-hour journey aboard a Jaffna-bound cargo ship. A government spokesman said another 400 would leave tomorrow.

But refugees in the Hindu College camp, who asked that their names be withheld, said they are afraid rising ethnic tensions in Colombo will overtake the government's slow progress in moving them out of the capital.

"Why can't they can't they get American ships, or British ships to take us away from here?" asked a Tamil businessman who is in the camp with his wife and two children. "In Lebanon, all the countries sent help. But Tamils are being massacred, too, and the government is not getting the ships."

For one 30-year-old refugee, the Sinhalese attacks that began Sunday after the ambush-slaying of 13 Sinhalese soldiers near Jaffna by Tamil guerrillas was accompanied by what he said was a terrifying sense of deja vu.

Attacked and burned out of his Colombo home by Sinhalese mobs in 1977 communal riots, he spent three days in a makeshift refugee camp and then was taken by ship to Jaffna, where he had relatives.

He said he spent two months in Jaffna but returned here because he could not find work. On Tuesday, the apartment in which he and his wife lived was attacked and burned and he fled to the same complex of refugee camps in which he had stayed six years earlier. He has asked to be shipped again to Jaffna.

"I worried that it would happen again but I had to come back," he said. "There was no work in Jaffna." Asked if he would return to Colombo again, he said, "If there are no jobs I'll have to, at a risk. But not with the family. I may come back alone for work."

In the Pillaiar camp yesterday, 4,000 men, women and children were attempting to feed and shelter themselves as best they could without any tents and little cooking equipment.

Several hundred women and children were crowded onto the floor of a large meeting hall in the walled temple complex, but most of the refugees simply slept on piles of rags in the dusty courtyards or milled around the complex awaiting word of the transport ships.

An outdoor water trough with five spigots was functioning, but refugees said they were afraid that inadequate sanitary facilities could lead to an outbreak of cholera.

International charitable agencies have been distributing food from the backs of trucks. CARE-Sri Lanka said it is distributing to each refugee in Colombo 10 biscuits a day and six ounces of thriposha, a high-protein ready-to-eat food made with U.S. commodities. CARE said it is also distributing food supplied by the Sri Lankan government and clothing material for 2,000 refugees.

With little to do during the day, the refugees spend their time talking about the elusive transport ships, their future and their experiences over the last week. A hardware store employe waiting to go to Jaffna said his Sinhalese neighbors with whom he had been friendly for years burned his house Monday and he and his family scaled a wall and ran to the safety of the camp.

"How can we live with them the Sinhalese now?" he asked. "We are prepared to live and cooperate with them, but when people do this to us, we have no choice but to go away and take our separate ways."

A deep-rooted animosity and distrust between the predominantly Hindu Tamils, who make up 20 percent of Sri Lanka's population of 15 million, and the predominantly Buddhist Sinhalese goes back for centuries to the influx of Tamils from India into Sinhalese kingdoms. The Tamil population is about evenly divided between those whose ancestors have been in the country for centuries and descendants of Tamils brought from India in the 19th century as plantation workers.

The hostility stems not so much from religious and language differences as from the Tamils' economic successes in a traditionally poor country and their ability to control a disproportionate segment of the industrial and mercantile sectors and live more comfortably in an inflation-ridden society.

Exacerbating Sinhalese resentment has been a growing Tamil separatist movement in the north and frequent terrorist attacks by a radical guerrilla organization, the Tamil Tigers, that advocates a separate nation.

On the main highway between Colombo and Kandy, 75 miles to the north, mobs of Sinhalese vigilantes roamed today, reflecting the sense of panic that has been sweeping the capital about rumored "suicide squads" of Tamil guerrillas moving toward the city.

In at least 10 villages, Sinhalese set up roadblocks to search cars and buses for Tamil passengers. Carrying heavy wooden staves, knives and rocks, Sinhalese youths stopped and surrounded the vehicles, demanding the identity of the passengers in an attempt to single out Tamils by whether they spoke Sinhala with a Tamil accent.

In Kandy, capital of the last Sinhalese kingdom until it fell to the British in 1815, Colombo Street, the main commercial thoroughfare, was almost entirely destroyed from the recent rioting. Scores of Tamil-owned shops, some of them still burning, had been ruined and their charred contents strewn into the street.

Most of the damage occurred Tuesday night, T.P.F. De Silva, police deputy inspector general in charge of the central region of Sri Lanka, said. He said that about 50 houses had been destroyed and two persons assumed to be Tamils were found floating in the picturesque lake in the town where once there was an island harem of a Sinhalese king.

A third fatality occurred on a tea plantation outside the city when, De Silva said, a Sinhalese was killed in a Tamil reprisal attack following a raid on Tamil plantation workers.

De Silva warned that while he hoped the worst is over, the security situation in the lush, tea-producing highlands northeast of Colombo is like a "soda bottle that can fizz over any time."

"The tension is there," he said. "The Sinhalese think the Tamils are going to attack them and the Tamils think the Sinhalese are going to attack them."

De Silva said 6,000 Tamil refugees were housed in seven camps in the area. He said there had been numerous cases of arson and looting in Nawalapitiya, a city of 200,000 persons south of Kandy.

In Peradeniya, a small town south of Kandy, at least 20 shops had been gutted in what appeared to be a pattern of arson aimed at Tamil merchants. Burning cars smoldered along the highway and smoking rubble littered the sidewalks.

In dozens of villages and towns along the main Colombo-to-Kandy road, the scene was repeated.