Mayor Vincent A. Cianci Jr. delivered his inaugural address behind a bulletproof screen on the City Hall steps today while 12 police sharpshooters watched from upper floor windows of six downtown buildings.

Another 100 policemen, half of them in chocolate and tan uniforms and the rest in plain clothes, mingled with the sparse crowd of 200 gathered in the city's windswept Kennedy Plaza to hear the speech.

The measures were taken because police are "taking very seriously" recent death threats against the mayor and Police Chief Angelo P. Ricci, said Maj. John J. Leyden, who headed the security operation.

Ricci said the recent assassination of Mayor George Moscone of San Francisco was a factor in his department's decision to provide such intense security.

Leyden said security at the noon inauguration was the most extensive ever provided in the city, with the possible exception of protection for President Carter during a visit here last year.

Police received five telephone death threats against the mayor and the police chief between Dec. 21 and Last Friday, Leyden said.

The caller said the two would be killed unless they resigned by New Year's Day, and blamed the mayor for the Jan. 16 death of Col. Robert Ricci (no relation), police chief at that time. Ricci killed himself with his service revolver in his office.

After Ricci's death, several police officers charged that he was driven to suicide because he had been forced to accept unqualified police recruits by Ciani. The mayor denied the allegation.

Last year was a rough one for Cianci. The mayor denied the allegational magazine which has since folded, reported that the mayor had been accused of raping a woman at gunpoint 12 years ago while he was a student at Marquette University law school. The mayor, who was never arrested or charged with a crime, has consistently denied the allegations, and says he will sue the magazine.

The city's financial health has also been a problem, Providence had a small surplus when Cianci took office four years ago, but now has a deficit of $7.5 million and faces a possible reduction of its high bond rating.

The mayor said in his inaugural speech that his second term will begin with an austerity program.