In a daring morning operation executed with military precision, a band of ultraleft commandos killed one policeman and bombed the Rome headquaters of the ruling Christian Democratic Party today, police here reported.

Two other police officers were seriously wounded in the attack, which had the markings of a Red Brigades operation. No group claimed responsibility for the raid, however.

The terrorist attack caused widespread concern here that the general election campaign that opens Friday may be a bloody one.

Shortly after 9:30 a.m., a group of about 15 guerrillas broke into the party headquarters, handcuffed office workers and planted five bombs, shouting, "This is a proletarian action."

Three of the bombs exploded, causing serious damage to the six-story building and shattering windows throughout the area. When a patrol car arrived on the scene, the machinegun toting attackers opened fire, killing Antonio di Meo, 33, and wounding the two officers.

The attack, described later by police as "a real military operation," set off a manhunt through the crowded streets of downtown Rome. Police and firefighters rushed to the scene.

Christian Democratic Party officials, government leaders and bystanders crowded into Nicosia Square in the heart of old Rome to survey the extensive damage.

This evening several thousand people demonstrated in two Roman piazzas to protest the continuing political violence. In one of the rallies Premier Giulio Andreotti told a crowd calling for stronger laws against terrorists that "the death penalty has never been the solution for a democratic country."

Party Secretary Benigno Zaccagnini sharply condemned the attack, saying, "Terrorism will not succeed, must not succeed, in transforming an electoral contest into a war." Similar messages poured in from political and labor leaders from a wide range of political groups, including the Communists.

Before leaving the party office, the terrorists used cans of red spray paint to cover the walls with slogans reading, "We will transform the fraudulent elections into a real class war."

With only a month to go before Italians go to the polls June 2 and 3 to elect a new national parliament, today's attack touched a nerve. Since the current Italian political crisis began two months ago, there has been concern that terrorists would take advantage of the leading to the elections to renew their attacks.

In the last two weeks there have been scores of minor bombings and Christian Democrats have been shot in the legs in Turin and in Genoa.

The Christian Democrats, who have governed Italy without interruption for the last 35 years, are basing their campaign on a plea for more votes to enable them to form a government without the powerful Communists, who are asking for posts in a future Cabinet. Pools here indicate that the Christian Democrats will make modest gains and the Communists, who at last count controlled 34.5 percent of the vote, are expected to suffer small losses.

So far no group has claimed responsibility for the attack.But the terrorists sprayed on the walls the five-pointed star that is the symbol of the red brigades as well as the letters, "BR," that organization's initials in Italian. CAPTION: Picture, A policeman salutes in homage to a colleague slain during an attack on the Christian Democrat office in Rome. AP