The public school system of Chicago will open its doors Wednesday to 520,000 students, but the city's attention will be riveted on 572 elementary schoolchildren who will be bused to a handful of neighborhood schools under a voluntary desegregation program.

Amid veiled warnings of "spontaneous" demonstrations and a threatened one-day boycott of schools in predominantly white southwest Chicago neighborhoods, the police are planning extraordinary security measures including the use of more than 800 officers from special operations units and an additional 1,200 on standby duty.

A police officer will ride on each school bus and squad cars will escort all of the buses from the predominantly black schools to the receiving schools.

Although the busing plan involves relatively miniscule numbers of student transfers - as few as half a dozen children at some schools - it has dominated the news here for weeks. Half the front page of today's Chicago Sun-Times was devoted to a photograph of scores of heimeted policemen charging menacingly across Soldier Field in a crowd-control exercise.

The plan also has generated impassioned hyperbole in a city whose sharply defined ethnic neighborhoods are still, for the most part, segregated.

A group of women in the Bogan neighborhood in southwest Chicago, defiantly calling itself "Bogan's Broads," has charged that radicals on the board of education and black militants have implemented a busing plan that, they claim, is by its nature a violent disruption of stable neighborhoods.

While professing to advocate nothing more than a one-day boycott, the leaders stress that they cannot predict what "the community" will do when schools open or what provocation "outsiders," may cause.

They noted that leaders of the Rev. Jesse Jackson's Operation PUSH program promised to provide protective escorts for black students transferring to white schools.

Moreover, the Rev. John Willis, head of the Martin Luther King Jr. movement, said today that black children should not look for trouble but warned that they must be prepared to protect themselves.

The focus of the attention is a small-scale busing plan that is little more than a modest extension of a 13-year-old plan to relieve overcrowding in the schools.

Under two segments of the plan, students from overcrowded schools may transfer to schools with unfilled classromms, receiving public transit tokens for their transportation.

It is the third segment - allowing elementary schoolchildren to move by chartered bus from "critically overcrowded" schools to less-crowded white schools - that has created the furor.

Of the 2,180 students eligible for the chartered busing program, only 572 had signed up by today. Of the 6,573 students eligible to switch under the long-running transit token program and the chartered bus program combined, only 1,184 had signed up.

School board officials today said they expected the busing to occur uneventfully

"It's a tempest in a teapot. The transfer has worked successfully for years in the north neighborhoods, but a gaggle of women in the southwest have tried to turn this into a crisis," said Thomas Maloney, an aide to school superintendent Joseph Hannon.

In the Bogan area, protest leaders today denied trying to provoke confrontations, but candidly admitted deep-seated fears about black intrusions into traditionally white neighborhoods.

Lorraine Black, chairman of Mothers for Neighborhood Schools American-Style, said, "It's destroying another neighborhood and destroying more of our schools. The only purpose of this is to integrate.It's not to relieve overcrowding at all."

Connie Schaefer, chairman of the Bogan Community Council, said that even though only 12 blacks are scheduled to enter one elementary school and 15 another, "it's not a matter of whether it's one or 100; it's an attempt to condition us to full mandatory busing in 1978.

"We have 24 per cent white students in Chicago and we're losing 10 per cent a year. We're going to have another Washington, D.C."

She said Bogan is unique because there is no buffer between adjoining all-white and all-black neighborhoods, as in north Lake Shore nieghborhoods, and that ethnic change "can happen overnight." Western Avenue, a dividing line to which she was referring, is black and Hispanic on one side of the street and all-white on the other.

Schaefer said antibusing leaders plan to take a bus tour Wednesday of the adjoining black neighborhoods, which they freely admit they have never visited. They have asked for police protection. She said blacks have "come here to look over their new schools, so we're going there to see what kind of schools they have."