At a streetcar stop in a working-class district of Turin, bloodstains on the sidewalk are only partially hidden by the flowers left by passersby mourning a 53-year old policeman, the latest victim of Italy's Red Brigades.

Several miles away, on the other side of this northern industrial city, 49 alleged members of the country's most notorious political group are about to go on trial in a specially constructed courtroom.

The Red Brigades trial and the terrorist phenomenon present a particular challenge both to the Italian judicial system and to Turin's Communist government.

There is increasing concern in Italy about the growing support terrorists are getting form thousands of disillusioned, unemployed young people who are scornful of all traditional Italian political parties, including the Communists, who have been in power in Turin since July 1975.

The Friday murder of police marshal Rosario Berardi has already left its mark on the trial.One of six jurors developed a mysterious illness and was excused from duty. A court-appointed attorney refused to take part, bringing to 56 the number of lawyers who have declined to defend the terrorists since the trial began.

The trial's first session in May 1976 was postponed for procedural reasons, but not before a commando had murdered Francesco Coco, the chief prosecutor of Genoa, and his two bodyguards. A second trial in May 1977 was called off when Fulvio Croce, president of the Turin Bar Association, was shot to death and half the jury members backed out.

The current session of the trial, now due to resume Monday, was also marked by difficulties in selecting a jury. More than a hundred names were called before 14 volunteers could be found.

About 4,000 policemen, mostly in Plainclothes, were brought into the city for the trial. At the courthouse, a converted army barracks, members of the press and relatives of the accused - only 15 of the defendants are in custody - were subjected to intense security clearance measures.

The trial has reawakened the concern of many Italians, particularly the citizens of Turin, of whether it is possible to check terrorism here before it does irreperable harm to Italian life.

The slaying of Berardi, a former member of Italy's antiterrorism squad, was the third in Italy so far this year attributed to the Red Brigades and the fourth in Turin since last March.

The self-proclaimed revolutionaires have turned from break-ins, arson, kidnaping and robbery to shooting victims in the legs and murder to make their point.

Nevertheless, despite Italian headlines describing a "city of fear" and Turin "in panic's grasp," the city's residents have been going about their business as usual.

Still, the residents of Italy's third largest city are shocked and worried by the spiraling terrorism that has become a daily fact of life.

Turin's Communist Mayor Diego Novelli points to an antiterrorism petition signed by more than 200,000 people as a signal that Italians are not likely to give in to terrorist intimidation.

But he does fear that if it goes unchecked, further terrorism could "unravel the fabric of Italian society and jeopardize its democratic institutions."

This opinion is shared by the head of the city's antiterrorism unit, who believes that although the hard-to-infiltrate Red Brigades are too few to pose a numerical threat, "things could become dangerous if Italians get so frightened they demand tougher and more repressive laws."

"If Italians get to the point where order is their major concern," said Christian Democrat City Councilman Maurizio Puddu, "they could turn to an authoritarian government."

Seen largely as an outgrowth of the turbulence that began with the student and labor protests of 1968 and 1969, terrorism in Italy reached a record high last year with police recording 2,128 separate terrorist acts.

Although other cities have experienced more violence, 15 months of bloody and "selective" terrorist violence has brought Turin into the spotlight. Red Brigades' murder victims in 1977 included lawyer Croce, another policeman, and the deputy editor of the Turin daily, La Stamps. In addition, ten other persons - journalists, politicians and Fiat executives - were blamed in shooting attacks.

"People are starting to feel insecure," an aide to a top Rome politican said. "They know how to deal with economic problems, but not with the fear of being physically harmed without knowing why."

Businessmen, politicians and editors who see themselves as targets now often carry arms. One well-known columnist still talks about the bruises he got after bumping into a top chemical industrialist who was wearing two pistols at his waist.

A high-level Fiat executive says he no longer leaves his home or gets out of his car without carefully looking around, and a top editor in Turin has a 24-hour police escort.

Newspaper editor Arrigo Levi points out that in 10 years no Italian government has succeeded in inspiring confidence to stem the disaffection that breeds terrorism.

"What this really means," Levi said, "is that Italy is a very sick country."