Officials from every branch of government and the Archdiocese of Boston are bracing for an expected influx into the city of up to a million people eager to catch a glimpse of Pope John Paul II on his first stop in the United States Oct. 1.

"This will be the biggest crowd event in Boston since George Washington landed on Dorchester Heights," said Capt. John O'Brien, operational commander for the state's metropolitan district commission police.

Plans were announced this morning to muster a security army of several thousand Secret Service agents, national guardsmen and city and state police.

The archdiocese of Boston, the nation's second largest Roman Catholic jurisdiction behind Chicago, presides over about 2 million Catholics. Catholics number about 5.6 million in a New England population of 14 million.

"We anticipate quite a mess of people; we're expecting an awful lot of people from Canada as well as New England," said Frank Sidlauskas, a spokesman for the archdiocese. "New Britain, Conn., alone is sending up 11 buses -- if that's any indication of the interest, then watch out."

The pope's itinerary for his 18-hour visit here calls for an opening reception of Logan International Airport, where he will disembark after a three-day tour of Ireland. He is expected to be welcomed by Vice President Mondale.

The Pope will then travel in an open car to the Cathedral of the Holy Cross in the city's South End neighborhood, where the 2,000 priests of the Boston archdiocese will attend a prayer service.

The pope is then scheduled to travel to the Boston Common, a sprawling park area in the heart of the city, expected to be jammed with about 400,000 people, where at about 4:30 p.m. (EST) he will celebrate a two-hour mass from a specially constructed $100,000 platform.

The mass will be celebrated with the aid of a 150-member chorus and about 20 musicians from the Boston Symphony Orchestra, church officials said. Forty to 50 cardinals and bishops will help concelebrate the mass, while 150 to 200 priests are expected to minister communion to the worshipers.

The city will position extra speakers to carry the mass and the Pope's homily, expected to be on the theme of youth, to the fringes of the common where the view of the platform is blocked.

Officials also plan to put up closed-circuit television screens in two or three sites in the city's downtown section.

Thousands of people are expected to jam the pope's motorcade route, which sources anticipated will wind down Commonwealth Avenue past Boston University and Boston College, two of the region's biggest schools.

That route stretches from the common to the Cardinal Humberto Medeiros' residence, where the pope will spend the night.

The pope will travel by helicopter on the morning of Oct. 2 to Logan Airport for a flight to New York to address the United Nations. He is also scheduled to visit Philadelphia, Des Moines, Chicago and Washington, where he will meet with the president in the White House on Oct. 6.