Jimmy Carter's inaugural walk up Pennsylvania Avenue was an effective piece of political symbolism, a deliberately informat act by a would-be President-as-plain-citizen. But don't think of it as a simple stroll among the populace.

For this year's inaugural procession, as for every other in grim modern history, precautions were immense. It may not have looked like it, but the President was closely guarded.

The D.C. police force has 4,300 men and women in its ranks, and 4,000 of the them were assigned to the 1 1/2-mile-long parade route.

Also deployed at the starting point of the parade and along its flanks were about 1,100 U.S. Capitol Police, an undisclosed number of Secret Service agents and scores of guards in the government office buildings along the avenue.

The Secret Service inspected the sewers beneath the parade route Thursday morning and the D.C. force had men equipped with radios, patrolling the highest rooftops, overlooking the roofs below.

All days off were canceled in the D.C. department. Only emergency leaves were granted, and some officers - from plainclothes squads such as narcotics - wore uniforms for the first time since they were rookies.

In addition to the lines of uniformed D.C. officers, who formed a kind of cordon along the parade route facing the crowd, there were several hundred plainclothes detectives in the crowd.

In the rest of the city, police services were maintained by the 300 membes of the D.C. force not assigned to the parade route, augmented by 850 D.C. National Guardsmen.

The guardsmen - some in jeeps, some in police scout cars, some on foot - took over a broard range of tasks, from patrolling beats to directing traffic.

Crime was down in the city as a whole Thursday, at least during the daylight hours. As of 4 p.m., about when the parade was ending, only 61 so-called Part One, or serious, offenses had been reported, which Police Chief Maurice J. Cullinate said compared "favorably with the day before," which he called "a low-crime day."

Eight arrests were made along the parade route. Five were for vending without a license, one was for possession of marijuana, and two were on robbery charges against suspected pickpockets.

There were two incidents involving the parade itself. In one, a man darted off the curb toward Carter as he walked by Sgt. Willie Doster, an 11-year police force veteran, had previously noticed the man, reading a bible and behaving strangely, and had assigned a policewoman, Addie J. Allen, to watch him. When he darted out, she had two Secret Service agents quickly tackled him. The man, whose name was not available yesterday, was questioned and released.

One D.C. police official also noticed what he thought was an unauthorized parader, a large-scale Abe Lincoln on roller skates, and made inquiries about him. But the man was past the presidential reviewing stand by the time that happened, and no action was taken.

D.C. police had not been told before rehand that Carter intended to walk the whole route, though the Secret Service had been informed three weeks before.

A Secret Service spokesman said the President's decision to walk instead of ride produced only "adjustments" in plans, not a major increase in security.

For one thing, the inaugural time and parade route are set in advance by tradition and widely known, so security precautions were already near the saturation point. And though no new President had ever led his entire inaugural parade on foot, some Presidents have left their cars and mingled with the people now and then. "So you have to be ready for that in any event," the spokesman said.

All in a day's walk, you understand.