The day that no one wanted and few believed possible-the day the nuclear power era would drive people from their homes-arrived here today.

As the crippled Three Mile Island atomic power plant near here continued to leak radiation, Middletown and sister communities began small-scale evacuations of preschool children and pregnant women.

The picture was mixed-small, peaceful towns dating to Revolutionary times, beset by bursts of confusion, occasional fear and self-conscious black humor.

Emergency vehicles toured, warning residents to take cover. Workers scurried about with bright yellow Geiger counters checking for radiation. Telephone communications jammed as circuits overloaded with calls. Banks were shut down. Some families hurriedly tossed suticases into car trunks. Others stayed. Police reported several accidents along main exit routes. Tonight's Middletown High senior prom was canceled.

By evening, officials said abou 150 persons were evacuated in buses to a sports arena at Hershey, 12 miles north of here. Extra police were brought into Middletown, a borough of 11,000, where the mayor said he would order his 13-man force to shoot looters.

"I know that won't go over well with people, but that's the way I feel," said Mayor Robert Reid. "When you're fooling with the atom, you've got to take it seriously."

While many of Reid's constituents went to Hershey, others went to homes of friends and relatives outside the area. At Falmouth, a village south of the nuclear power plant, no one was on the streets at mid-afternoon. One-fourth of the Goldsboro community's 600 residents had left.

The evacuation was suggested around noon by Gov. Richard Thornburgh, who said he believed "an excess of caution is best" even though radiation leakage was reported to be no more severe than it was when the plant went out of operation early Wednesday morning.

Thornburgh, who refused to term the evacuation an evacuation, ordered the closing of 23 schools within a five-mile radius of the plant, located on a Susquehanna River site, and "advised" that young children and pregnant women-those most vulnerable to radiation effects-leave the area.

Schools and residents in portions of three counties-Dauphin, Lancaster and York, with total population of about 815,000-were covered by the governor's suggestion.

Thornburgh's advice and contradictory statements from the Metropolitan Edison Co., operator of the crippled plant, continued to confuse and confound many who complained bitterly about sketchy and uncertain information.

At a contentious press briefing here, company vice president John Herbein said, "We don't believe an emergency exists to evacuate . . . I'm here today to try to lower the level of panic and concern."

Herbein disputed the governor's data on radiation levels outside the plant. A radioactive emission at the plant this morning, Herbein said, was not "uncontrolled," as Thornburgh had described it. The release of radioactive steam into the atmosphere was planned and controlled, Herbein said.

"I don't know why we need to tell you every step we take," Herbein told reporters. "We certainly feel a responsibility for people who live around our plant and we need to get on with our job."

But the mayors of Middletown and Royalton, two Susquehanna River communities that lie in the ominous shadows of the plant's huge cooling towers, took strenuous exception.

"The same old malarkey," said Middletown Mayor Reid, after hearing Herbein. He was still rankled from Wednesday's episode and a call earlier today from a company official.

On Wednesday, after an emergency was declared at the plant three miles down river, Reid said the company told him there was "no problem." Then he learned radiation had leaked.

Today he was told the plant had lost some steam because of pressure building in the nuclear reactor container. Reid said no mention was made of radioactive emissions.

Royalton Mayor Charles E. Erisman, whose town of 1,040 is two miles from the plant, said that as of this morning he still had had no contact from power plant officials explaining their problems.

"I am concerned. We should have been notified. About 75 percent of our people are retired and half of them have no out - no transportation of their own," he said.

By mid-afternoon, the residential section of the two towns took on a ghost-like air, made eerier by the glint of sun through a heavy mist cover.

Fire and police vehicles with loud-speakers, dispatched into the streets by Middletown Mayor Reid, were urging residents to leave. Firefighters went door to door, warning people to leave if they wanted.

The driver of one fire truck, a young employe at the Three Mile Island plant, said he felt the nuclear station was safe and that evacuation was not necessary.

Nonetheless, he followed his mayor's instructions, and residents were leaving quickly.

But here, as in Harrisburg, the state capital about 15 miles upriver, confusion and uncertainty were the orders of the day. No one really knew just how serious the situation was at Three Mile Island.

Shortly before 10 a.m., civil defense officials in the capital urged residents of a five-county area within a 10-mile radius of the plant to go indoors, turn off air conditioners and close windows.

That advisory was issued aftr Metropolitan Edison officials notified the state that a small amount of new radiation had been vented from the nuclear plant.

The news from the utility left employes of the state Emergency Management Agency and the Public Utilities Commission visibly upset-they apparently had not expected more venting of radioactivity.

Half an hour later, tension in Harrisburg heightened when an air raid siren whined for about a munute. It was apparently a mistake, since no emergency had been declared.

James McDonald, a city hall adviser, called it "just a dumb accident," but the alarm prompted early departures from state and city offices. By mid-afternoon most offices were barely operatonal-employes had left to join families and friends.

State Rep. Joseph Rhodes, a Democrat from Pittsburgh, sent his office staff home early. Why? Rhodes looked at Thornburgh, who was presiding at a news conference, and said, "because what they're telling us is bull . . . I don't believe them."

House Speaker H. Jack Seltzer, a Lebanon County Republican, said he would not comment on the developments or on the conflicting reports.

"Look," said Seltzer, "I know how to make bologna and I know how to be a politician. But I don't know a damned thing about nuclear reactors. The best thing for us politicians to do is to do little until things cool down. Then we can go around and ask a lot of profound questions."

Back here in Middletown, Mayor Reid, deadpan, asked his own profound question:

"You want to be mayor?"