A powerful bomb exploded in a telephone booth of the main office of the Sri Lankan Telecommunications Department yesterday, killing 11 persons and wounding 114 others, according to news agency reports from Colombo, capital of the South Asian island nation.

Sri Lankan diplomats in Washington said that more than 50 of the injured are in serious condition and that the death toll is expected to rise.

Yesterday's bombing follows an explosion on an Air Lanka jet Saturday as passengers were boarding at the Colombo airport. At least 15 persons died in that blast, most of them European tourists. Two U.S. officials aboard the plane escaped unhurt.

The two explosions forcefully brought to the capital the bitter civil conflict that has left hundreds dead, mostly in the northern and eastern portions of the country, over the past several years as Tamil separatists have waged a campaign for greater autonomy or independence from the Sinhalese majority.

More than 75,000 of the capital's 600,000 residents are Tamils, and even the most militant of the separatist groups have been fearful of challenging the government on its doorstep out of fear of a backlash. Communal rioting swept Colombo in 1983, leaving about 400 dead, most of them Tamils.

Analysts who follow Sri Lankan affairs closely said yesterday that the Tamil guerrilla organizations have shown signs of splitting in recent weeks and militant splinter groups may have thrown off the inhibitions that had kept their operations limited mostly to the Tamil heartland.

The Tamils, who make up about 18 percent of the country's 16 million population, are mostly Hindus and have close ties to the Tamil population of the southern Indian state of Tamil Nadu and its capital, Madras. Training camps for Tamil guerrillas reportedly have operated from time to time in southern India. The majority Sinhalese are mostly Buddhist.

Many residents of Colombo rushed to their homes after yesterday's explosion at 9:23 a.m., apparently fearing a new outbreak of violence similar to that of 1983, Reuter reported. The explosion wrecked two floors of the red-tiled, whitewashed building, which is only 500 yards from President Junius Jayewardene's official residence. It was unclear whether Jayewardene was home when the blast occurred.

Witnesses said frightened citizens fled from the scene and debris was strewn over a wide area. The office of Post and Telecommunications Minister D.B. Wijetunge was heavily damaged in the blast, which occurred in the section of the building used for offices.

In a statement released in Washington, Sri Lankan Ambassador Ernest Corea said that a preliminary chemical analysis "has shown that the type of bomb used was identical with others detonated on numerous occasions by a Madras-based separatist . . . group, which calls itself the Liberation Tigers of Tamil Eelam . . . ."

Corea said the government has intercepted communications linking the bombing to the Liberation Tigers group, one of four principal Tamil groups that have been involved in running battles with government security forces for more than three years.

The Saturday bombing occurred as passengers were boarding a flight from Colombo to the Indian Ocean resort of Male in the Maldive Islands. The powerful explosion tore the Lockheed L1011 in two. The plane was running about 15 minutes late for departure, and officials said that had it taken off on time, all 126 passengers plus crew probably would have been killed.

Spokesmen for Tamil groups in Sri Lanka and in the United States have denied involvement in the two blasts. A caller to The Associated Press in London said that a Sinhalese militant group called the People's Liberation Front was responsible, but there were no similar reports from Colombo. There have been unconfirmed reports of contacts between Tamil fringe groups and the Sinhalese Front, an alliance that could prove troublesome for government security forces.

Sri Lankan officials immediately linked the two explosions to an effort by the Tamils to undercut a new round of talks between the Jayawardene government and the Indian government of Rajiv Gandhi, which has wavered between support for the Tamils' militant demands and a mediator's role.

Corea said yesterday that the airport explosion occurred only hours before an Indian delegation was to leave Colombo with a new set of Sri Lankan proposals for greater autonomy in the Tamil northern region around Jaffna.

Further complicating the dispute has been bloody internecine fighting between major Tamil groups. The Sri Lankan government has portrayed this fighting -- at a time when negotiations were under way -- as an effort by militants to block some groups from discussing compromise proposals, but other observers have said that the timing may have been coincidental.