Troops tightened a strict, 56-hour curfew on Sri Lanka's capital today, bringing an uneasy calm to the deserted streets but also increasing hardship for the city's nearly 1 million housebound residents.

The curfew--extending from 5 p.m. Friday to 5 a.m. Monday and imposed after 33 persons died in street violence yesterday--has led to panic and food shortages throughout the capital area.

The capital was calm for the most part today for the first time since the country's worst communal violence since 1948 broke out Monday, resulting in an official total of 108 killed. Unofficial estimates put the number of ethnic Tamils slain by mobs of Sinhalese at twice that number.

The government today revised its estimates of Tamil refugees left homeless by the six-day wave of arson and looting to 48,000 across the country, with 35,000 of them housed in makeshift camps in Colombo.

The food shortages here worsened as cargo ships lay at anchor fully loaded at Colombo ports because there were no longshoremen available to offload them.

An official at the docks said 35 ships were backed up, and that no cargo had been unloaded in three days, except for that taken off one vessel that was needed to transport Tamil refugees to the predominantly Tamil northern peninsula.

Before the curfew was reimposed yesterday, long lines stretching for blocks had formed at food stores throughout the city as residents sought to stock up on essential goods.

Rumors that Tamil guerrillas had infiltrated Colombo and were planning a major terrorist action in response to President Junius R. Jayewardene's decision to ban advocacy of Tamil separatism led Prime Minister Ranasinghe Premadasa to appear on television and promise that "our government has the strength to protect you and the country."

Referring to a constitutional amendment to be introduced in the Parliament--five-sixths of whose members are of Jayewardene's United National Party--Premadasa said, "No one would be able to talk about it . . . . We would not only deprive those advocating any division of the country of their civic rights, we would even bring legislation to confiscate their properties. Those advocating any division of the country will not be able to talk about it even in a foreign land because we would punish them upon their return to Sri Lanka."

The ban would effectively eliminate the Tamil United Liberation Front, the only moderate buffer between the government and the terrorist Tamil Tigers and, according to many political observers here, force the radical movement to adopt more extreme measures to keep its cause alive.

In a related development, the government charged today that the country's unrest was part of a plot masterminded by foreigners to oust the government, and it banned three Marxist groups: the pro-Soviet Communist Party of Sri Lanka, the extreme-left People's Liberation Front and the New Socialist Party.