The first public notice of the Soviet nuclear plant accident came after an alarm was set off this morning when a worker at the Forsmark Nuclear Power Plant, 800 miles away on Sweden's Baltic coast, was found to have unusually high levels of radioactive contamination on his clothing.

Further tests showed four to five times the normal level on the soil and on the leaves on trees outside the power station.

Seven hundred workers were evacuated from the plant and taken to a decontamination unit nearby. A radio warning was broadcast to local residents telling them to keep away from Forsmark, and police road blocks were set up to turn back motorists.

While emergency investigation procedures were being undertaken at Forsmark, reports of high radioactive levels began coming in from nuclear plants throughout southern Sweden and from the other Scandinavian countries.

Even before the Soviet announcement, Swedish authorities had ruled out the possibility of an accident here as the source of the radioactive isotopes. Expert opinion in Sweden was that the contamination and fallout being detected here probably had come from an accident at a reactor in the Soviet Union.

"It was immediately apparent that the radioactivity was not due to a nuclear weapons test because of the particular combination of certain isotopes such as cobalt, iodine and cesium," said Lars Eric de Geer, a radiologist at Sweden's Defense Research Institution.

Analysis here was said to point to a nuclear power plant out of control -- probably the cooling system breaking down and the fuel overheating and burning its way out of the reactor and polluting the atmosphere.

"It must have been a rather major catastrophe for us to measure such concentrations at so great a distance," de Geer said.

"Later analysis of our instruments showed that the radioactive cloud came over Sweden about 2 p.m. on Sunday, indicating that the accident occurred sometime before then," he said.

Officials at the Nuclear Power Inspectorate said that the "radioactive elements could have been carried upwards in air currents and then come down again over Scandinavia."

Defense officials are of the opinion that if no more radioactivity is dumped over Scandinavia, most of the airborne radioactive isotopes will soon disperse, while those contaminating the ground will take up to a week to decay.

Once it had been established that the contamination did not come from a Swedish or other Scandinavian plant, Swedish Energy Minister Birgitta Dahl contacted the International Atomic Energy Agency in Vienna, which then questioned several governments including the Soviet Union, according to sources here.

The Swedish Embassy in Moscow asked authorities there whether there had been an accident, officials here said, and initially received a negative reply.

Confirmation did not come until several hours later.

"I do not discount the possibility that our questions both directly via our embassy and via the IAEA led to the Soviet Union being forced to disclose what had happened," Dahl said. "We have now instructed our embassy in Moscow to request detailed information."

At a press conference late this afternoon, Dahl said she found it completely unacceptable that Sweden had been given no warning, and that many hours after the blanket of radioactivity had spread over Scandinavia, the government still had no real information as to a source.

"We shall now have to closely consider the political implications of this incident," she said.

Dahl said Finland first detected high levels of radiation on Sunday night, but had not informed neighboring countries.

A group called the People's Campaign Against Nuclear Power demanded here today that the government immediately shut down all Swedish nuclear plants until it was known what caused the Soviet accident.

Dahl rejected this demand, however, saying that the Swedish people had no cause to be concerned about safety standards at Swedish nuclear plants.

"What is needed, however, is that we now place demands on the Soviet Union and other countries for a higher order of safety," she told reporters. "We must also demand that in such situations one alerts one's neighbors and furthermore we shall reiterate our demand that the whole Soviet civilian nuclear program be subject to international control."

Dahl said contamination levels in Sweden at midday today were not high enough to be considered harmful to health. But she said it was not known whether the contamination was continuing or whether it would increase.

People here have not been asked to take any special precautions, but it was clear tonight that many Scandinavians intend to stay indoors Tuesday, at least until they feel they have been assured that it is safe to go outside.

The Defense Research Institute continued taking measurements both on the ground and at high altitudes tonight in order to gain a clearer picture of the possibly dangerous radiation concentrations.

A team has also been sent to Gotland, a Swedish island in the Baltic Sea, where "very high" readings have been reported. Air particles collected over the Baltic Sea are being analyzed by Swedish scientists.

The Soviet nuclear accident comes at an uncomfortable time for Sweden's Social Democratic government.

Several members within the party feel that the government is not truly committed to dismantling nuclear power in the country, a task that is supposed to be completed by 2010.

Some party members this week resigned in protest at what they called the government's "stalling tactics."