BOSTON--The Boston Archdiocese yesterday announced it will finance the construction of a $150,000 platform and altar that Pope John Paul II will use in an outdoor mass on the Boston Common Oct. 1.

The Massachusetts Civil Liberties Union objected to the city paying the cost of the Pope's visit, which city officials estimate at about $700,000.

"It would be a great pity if the public discussion of financial matters relating to the Pope's visit were to distract us from this unique opportunity," the Rev. William Helmick, the archdiocese's coordinator for the Papal visit, said yesterday.

The archdiocese also said it will pay the cost of the sound and electrical systems needed for the mass.

Last weekend, church officials called for special church donations in honor of the Pope's visit to Boston--his first stop on a six-city tour of the United States. The archdiocese said yesterday collections have been two to three times above normal.

Insecurity

Boston officials, who now estimate that more than one million people will line the Pope's motorcade route and attend the outdoor mass, said they plan the tightest security in the city's history.

About 7000 security officers, including 5000 national guardsmen and 500 state police troopers, will assist in crowd control along the 20-mile motorcade route, Boston Police Commissioner Joseph Jordan said yesterday.