Italian police stepped up a massive manhunt here yesterday for the 12 kidnapers of Italy's former premier Aldo Moro who was abducted Thursday after a brief but bloody ambush in which five of his police bodyguards were shot dead.

Primary efforts were concentrated in Rome's outlying Montemario area where Moro, 61, a five-time premier who is currently the president of Italy's ruling Christian Democratic Party, was seized. Thousands of police were used in the search.

After the discovery of a second blood-stained car abandoned by the Red Brigades kidnapers, the search was extended into other surrounding areas of Rome.

Police said they believed the bloodstains may have been left by a terrorist wounded in Thursday's shoot-out, but they could not entirely discount the possibility that Moro had been injured.

Concern was also expressed about Moro's general health. Two years ago, one of his adrenal glands was removed in surgery and he needs an injection every few hours.

According to police reports, as many as 2,000 apartments in Rome and 5,000 cars have already been searched. Thousands of local policemen, national police or carabinieri and coast guard units were patrolling other Italian cities and the country's frontiers on the chance that the dark-colored sedan that spirited Moro away had managed to leave Rome before an effective network of roadblocks was installed.

In Milan, an 18-year-old youth was reportedly shot to death when he failed to stop at a police road-block. Authorities said later that he apparently had no connection with the kidnaping.

Police sources said last night that "several persons" had been taken into custody but it was too soon for any comment on their possible involvement in the kidnaping.

The abduction of Moro, one of Italy's leading politicians and a likely successor to President Giovanni Leone, presented Italy's New Christian Democratic government with a major crisis only a few days after it was sworn in office.

Italy's major daily newspaper, Corriere della Sera, referred to the kidnaping as "the most serious political crime in the last 30 years." The Rome Daily, II Messaggero, described it as a threat to democracy, and the Turin daily, La Stampa, accused the terrorists of trying to show Italians that their country is ungovernable.

After winning a massive vote of confidence Thursday from parliament for Italy's first Communist-backed government in 31 years, Premier Giulio Andreotti yesterday held emergency meetings with key Cabinet members and top police and intelligence officials in an attempt to coordinate an effective investigation.

Police kept a tight lid on the investigation's process, but they reportedly were following leads offered by the 710-odd bullets left on the scene and by the airline stewards' uniforms worn by several of the terrorists. About 30 witnesses to the kidnaping also were being questioned.

The mood yesterday in Rome was somber as Italians waited for news of Moro's fate. Several telephone callers identifying themselves as Red Brigades have demanded the release of jailed comrades and the suspension of a Turin trial, scheduled to resume Monday, of 15 of the group's leaders.

One day after the trial opened, the Red Brigade murdered a Turin policeman who was scheduled to testify.

Hundreds of thousands of Italian workers returned to work yesterday following an all-day strike Thursday that ended with massive antiterrorism demonstrations in most major Italian cities.

In Rome, alone, an estimated 200,000 filled San Giovanni square Thursday afternoon. Yesterday smaller demonstrations were held in cities like Bari and Palermo, while here in Rome a group of so-called leftist, prisoners at Regina Coeli prison wrote to Premier Andreotti condemning the kidnaping.