Despite renewed opposition from leftist and pacifist groups, preliminary work has begun at Comiso in Sicily on the projected base for U.S.-built cruise missiles.

"We have made our choice," Defense Minister Lelio Lagorio, a Socialist, said in an interview. "There will be polemics and debates, but the decision has been made."

The government, while expressing confidence, seems to be making an effort to keep developments regarding the base as low-key as possible. Lagorio reaffirmed Italian support for negotiations for a possible deal in which NATO would halt planned installation of its cruise and Pershing II missiles in exchange for the Soviets' withdrawal of their SS20 medium-range missiles aimed at Western Europe.

One issue that everyone appears eager to avoid is who will decide whether to fire the missiles once they are in place. It is unclear how much power Italy will have over the missiles, and this represents a potentially explosive political topic.

On April 4 thousands of Sicilians, young people from other parts of Italy and pacifist groups from several other West European countries joined a large antinuclear arms march to the Magliocco military airport near Comiso, where the base is to be constructed.

The Italian Communists, who organized the protest, said 100,000 people participated, while the Italian news agency ANSA said 50,000 were present and the state television network estimated 40,000. It was the first major march against nuclear arms in Italy since several last October in Rome, Milan and other Italian cities.

Nevertheless, technicians from a consortium of local building firms, which was assigned the $1.1 million contract last month, began preparations for the first phase of work, mostly involving the demolition of World War II structures and construction of temporary housing for workers.

Italy first committed itself to stationing 112 cruise missiles on its soil in November 1979 as part of a NATO plan to deploy 572 medium-range nuclear missiles in Western Europe. The Italian decision was regarded as crucial by U.S. and NATO policy-makers because West Germany only agreed to accept its share of the weapons on condition that another European NATO ally besides Britain, which has nuclear forces under its own control, do so as well. West Germany will have more than a third of the missiles.

Lagorio, the defense minister, said that the government was likely to have fewer problems than other European countries with peace movements because "here, the protest is more or less monopolized by the Communists," who are in the opposition.

In contrast to demonstrations in other West European countries, neither the Roman Catholic Church nor any of Italy's five government coalition parties was involved in the Comiso march.

The string of demonstrations last fall ended after a few weeks. At that time some government parties such as the Socialists got briefly involved but were quick to give their protests a general pro-peace tone and not the anti-NATO, anti-United States thrust of the April 4 march.

Even the Communists' recent antinuclear activism, which is part of a current general antigovernment campaign, has been relatively mild. They allowed the 1979 decision to install the missiles to pass almost unnoticed and only began organizing protests last fall after demonstrations had already spread through much of Western Europe.

Italy's antinuclear groups, which include several small leftist parties, tiny environmentalist groups and the Catholic Workers' Association are planning several other protests including a large demonstration in Rome timed to coincide with President Reagan's scheduled visit here June 7.