COLOMBO, SRI LANKA, JULY 28 -- Hospital authorities today reported that 18 persons were killed and more than 100 injured as police clashed in downtown Colombo with rioters protesting a proposed political settlement of this island's ethnic strife.

The sources reported that most of the deaths appeared to be the result of police firing on mobs as they stoned and torched dozens of government buses and police vehicles.

Officials imposed a curfew on this capital of almost 1 million tonight in anticipation of Wednesday's arrival of Indian Prime Minister Rajiv Gandhi, who intends to sign the proposed peace accord with Sri Lankan President J. R. Jayewardene. Officials indicated that the curfew would be extended to cover the entire period of Gandhi's visit, which is to end Thursday morning.

The accord to be signed by the two South Asian leaders is designed to end four years of civil strife between this nation's Tamil minority and its Sinhalese majority. An estimated 6,000 people have died since the conflict intensified in 1983.

Sri Lanka's Tamils are mostly Hindus who share close ties with the Tamil population of southern India. They have enjoyed the support of Indian officials in their battle for increased recognition. This has given New Delhi greater influence in the affairs of its small southern neighbor, enhancing its role as the potential broker of a Sri Lankan truce.

But so far, New Delhi has not been able to persuade the main Tamil guerrilla group, the Liberation Tigers of Tamil Eelam, to go along with the proposed agreement. In the absence of cooperation from the guerrillas, Jayewardene is now facing a revolt not only from his foes among Sri Lanka's Sinhalese majority but also from his own Prime Minister Ranasinghe Premadasa.

Today's violence began when supporters of the Sri Lankan Freedom Party of former prime minister Sirimavo Bandaranaike and Buddhist monks gathered for a rally near Colombo's central Pettah district. As the rally ended, according to witnesses, crowds began to attack government-owned buses, police vehicles and other symbols of authority. Soon black columns of smoke dotted central Colombo and spread into several residential areas.

Near the Fort railroad station, the city's main rail terminus, police trucks were commandeered and burned as their occupants reportedly sought safety in nearby buildings. One truck, its windshield smashed, was seen careening from the area around broken fences strewn across the wide boulevard that runs past the station. In the back of the station, policemen indiscriminately fired shotguns at the surging crowds. Clouds of tear gas filled the air, mixing with the black smoke of burning vehicles.

For much of the midmorning the streets around the railroad station belonged to the rioters and demonstrators, most of whom shouted epithets against Jayewardene and voiced opposition to the proposed accord.

The intensity of the demonstration clearly caught government officials by surprise. "We expected trouble, but not on this scale," said one official.

Nevertheless, a Foreign Ministry official said, "we are determined to go ahead" with the Gandhi visit and the signing of the peace accord. "There is a determined decision to go ahead. This is a stray incident. It is just vandalism," he said.

Sri Lanka's Sinhalese, who are mostly Buddhist, argue that the proposed accord goes too far in giving the island's Tamil population its demand of an independent homeland. Premadasa, among others, has vowed to fight its implementation.

Tamils, who are mostly Hindus, make up about 18 percent of Sri Lanka's 16 million people. They claim that the Sinhalese, who control the government and the Army, discriminate against them.

Under the proposed agreement, the Tamil guerrillas are to lay down their arms under a cease-fire and amnesty and the government will agree to a single provincial government for the island's Northern and Eastern provinces. The Northern Province is predominantly Tamil, while the Eastern Province is split among Tamils, Moslems and Sinhalese.

The proposal for a single provincial government, however, appears to concede a primary Tamil demand previously rejected by the Sri Lankan government.

A proposed referendum in the Eastern Province on the joint government is one of the provisions opposed by Velupillai Prabhakaran, head of the main Tamil guerrilla group. He also is said to be balking at plans for an early surrender of his organization's weapons.

His objections have caused previous peace negotiations to collapse. India and Sri Lanka reportedly have been close to agreement on a peace accord before, only to see the agreement founder when the Tamil guerrillas apparently refused to comply.