Government specialists reported here today that levels of radiation in Poland were as much as 500 times higher than normal Monday and that while they had sharply receded by today, the country could expect an increase in cancer rates of "several percent" in the next two to three decades.

In the first detailed report by authorities of the effect of the Soviet Union's Chernobyl nuclear plant accident on Poland, members of a government-appointed commission said at a press conference that there was a "radical reduction" of traces of radioactive iodine in the atmosphere since yesterday -- putting them about the same level as Sweden -- and that Polish monitoring confirmed that emissions from the Chernobyl plant had ceased.

However, the commission members advised Poles to continue observing safety guidelines announced earlier in the week, including not drinking milk from grass-fed cows, and reaffirmed that the estimated 11 million Poles under 16 years of age would all be given doses of sodium iodide to help prevent contamination.

Polish families continued to pick up doses of iodine at health stations around Warsaw today even as the government staged a four-hour May Day parade and its opposition countered with a mass attended by about 5,000 at a Warsaw church. Long lines and scenes of panic that marked the medicine distribution in Warsaw yesterday did not recur today, but many families continued to keep their children indoors and closed windows despite warm, sunny weather.

Communist leader Wojciech Jaruzelski delivered a 15-minute address at May Day ceremonies but made no mention of the nuclear accident. Around the country, authorities appeared more preoccupied with preventing opposition demonstrations than countering the radiation threat.

Thousands of militarized police were deployed around churches in Warsaw, Krakow, Gdansk, Poznan and other cities where opposition masses were held. Though no marches or clashes were reported, police detained at least 25 persons near the St. Stanislaw Kostka church in Warsaw, beating several who ignored orders to disperse.

At the government commission's press conference, Zbigniew Jaworowski of the Central Laboratory of Radiation Prevention said there was no immediate health danger to Poles and that the recorded radiation levels were similar to those recorded in neighboring Sweden. However, he said that an assessment by experts had concluded that "we can expect in the next 20-30 years that there will be an increase in cancer of the thyroid and cancers of other organs of the body."

Jaworowski said he expected the increase to be "a few percent," an amount he said was "minimal." Steps taken by the government, he said, could "prevent our children from illnesses in 15 or 20 years -- but that is only our wish."

Mieczyslaw Sowinski, the chairman of the state atomic energy commission, confirmed that the accident at the Soviet plant had occurred on Saturday and attributed it to "a leak in the construction of the building" that caused "radioactive substances to escape outside."

Sowinski said that measurements in Poland showed radiation reaching Poland between Sunday and Monday. Jaworowski said the highest level, 2.5 milli-roentgens per hour, was recorded Monday morning in the town of Mikolajki, about 120 miles north and slightly east of Warsaw. In other regions, he said, readings showed radiation Monday only 10 times more than normal.

Sowinski commented that "it is hard for us to be free of any doubts" about what is happening at the Soviet plant. However, he said, Polish readings showed that the source of radiation "was one, and it has stopped."

The experts and government spokesman Jerzy Urban refused to answer questions on the sensitive subject of when Poland had been informed by Soviet officials about the accident.

The two-hour press conference appeared intended in part to answer criticism that authorities had been too slow to warn citizens about the radiation cloud and had not provided adequate information about levels of exposure.

Jaworowski said officials had delayed announcing specific radiation measurements because they wished to avoid the "chaos" and "hysteria" he said followed precipitous statements by U.S. officials following the Three Mile Island nuclear accident in 1979. "Ours was specific concrete information rather than sensationalism," he said.

Dependents of western diplomats began leaving Poland today because of uncertainty over the danger, and the British government advised its citizens not to travel to Warsaw or northeastern Poland.

A spokesman of the Canadian Embassy said a group of 12 dependents would leave for West Germany on a voluntary basis tomorrow, "because we can't get sufficient information both about the present situation and the future situation." The French Embassy advised dependents to leave Warsaw yesterday.

A U.S. Embassy spokesman said tonight that no embassy employes or dependents had left the country and they had not been advised to do so. However, he said, U.S. officials here had taken the precaution of importing bottled water from Germany for its staff.

Leaflets distributed by opposition groups sharply criticized the lack of official information and citizens interviewed around the city commonly expressed anger at the government.

"We never would have known about this if it hadn't been for third parties in the West," complained Eva, a young woman who brought her 1-year-old son, Pavel, to a city park this afternoon. "I'd like to know the reasons why this happened, and I'd like to know whether it's safe to bring my child outdoors. But in fact we don't know what's going on."

Several persons said they feared they had learned of the danger too late and had not been able to take steps to protect themselves and their children when the radiation levels were at their highest. "I wasn't able to help myself," said a young, pregnant woman in a park, "and now no one is able to help me."

Still, many Poles appeared to have retained an ability to laugh at their predicament. At a public cabaret show in Warsaw last night, jokes were rife about what one wit called "the cloud of friendship" in mockery of official propaganda about Soviet-Polish relations. "May you have a radiant May 1," a performer told the audience.