Miners abandoned a 13-day sit-in at a Silesian coal mine yesterday, ending the last major protest by workers against Poland's martial law, Radio Warsaw said.

The last 900 holdouts, some of them "very weak" from their two weeks underground, left the shafts at the Piast mine "voluntarily," the official radio said, and the entire work force at the mine was ordered to return to work today. There was no independent confirmation of the official account and a communications blackout imposed at the start of martial law remained in effect.

Elsewhere in the country, Radio Warsaw said, nearly all workers had reported to their jobs on the first workday after the Christmas weekend.

Other information, however, reaching Washington from Warsaw contradicted the Radio Warsaw account. Despite attempts by Poland's military authorities to restore a sense of normalcy, key factories around the country remained closed yesterday or were working at far below normal production, these reports said. Details on A10.

Radio Warsaw's contention that significant organized opposition to the two-week-old martial law has ended came as Poland's crisis of food and medical supplies worsened by the day, according to official accounts and private Western relief organizations.

A day after Polish authorities announced sharp new reductions in meat and butter rations, Western governments and relief agencies looked for ways to step up their shipments of food and medicines to the virtually bankrupt country--and to ensure that the supplies reached needy civilians rather than the military, which now rules Poland.

Five top officials of the International Red Cross in Geneva, flew to Warsaw yesterday to confer with Polish officials on coordinating aid "in favor of persons most affected by the current situation," The Associated Press reported. They brought with them four tons of antibiotics and other medical supplies destined for the Polish Red Cross and the Polish Health Ministry.

In Brussels, European Common Market officials said they were pressing Polish authorities for assurances of proper distribution that would lead to shipment of 8,000 tons of beef to Poland.

Ambassadors of the 10 EEC countries were told in a progress report on negotiations that the Polish government is expected to formally tell the commission in Brussels shortly that arrangements have been made for independent, nongovernment bodies to import and deliver the beef to the Polish people, the Manchester Guardian reported.

The assurances are expected to include Polish acknowledgment that the food is intended for "internal consumption by the civilian population only," the Guardian said.

Questions about distribution and use of the donated food and medical aid led to the Reagan administration's decision shortly after imposition of martial law Dec. 13 to suspend official U.S. food aid programs and to allow continued shipments of private American aid only if the relief agencies can be sure that it is reaching needy civilians.

President Reagan warned last Wednesday that private aid shipments will be allowed only so long as "we know that the Polish people themselves receive the food." At the same time, he said that U.S. shipments of food under an $800 million aid program would remain suspended "until absolute assurances are received that distribution of these products is monitored and guaranteed by independent agencies."

"We must be sure that every bit of food provided by America goes to the Polish people--not to their oppressors," Reagan said.

Joseph Carniglia, director of international services for the American Red Cross, said yesterday that the Polish government is assisting the Polish Red Cross in transportation and distribution of aid shipped by Red Cross societies. He added, "We have first-hand confirmation that the people who are supposed to get it are getting it.

Catholic Relief Services, which, with CARE, accounts for the major direct shipments of American relief to Poland, said yesterday that it has sent 16 shipments of food to Poland since July 17, totaling 8,500 tons valued at $10 million.

Spokesman Lynn Marshall said five more shipments are being prepared, with the next one--including 196 tons of butter and 30 tons of winter clothing--scheduled to leave on the Sikorsky, a Polish Line vessel, Thursday.

The aid is shipped directly to the Polish Bishops' Conference, Marshall said, "and we know through our church contacts there" that it is reaching people "and that they still need a great deal more."

Agence France-Presse quoted Catholic Church sources in Rome yesterday as saying that the Polish bishops have reached an agreement with the Polish government on the distribution of emergency food aid in Poland.

They said the two sides would cooperate in drawing up a list of the elderly, sick and children in each diocese who would get priority in food handouts.

Except for major cities where diplomats live, Western governments have no direct way of gauging the severity of the food shortages in Poland since most travel is banned.

Although Poland has continued to seek food from abroad, Radio Warsaw said yesterday that supplies of bread, butter, meat and poultry were adequate in the capital at the moment, but it gave no indication of the situation elsewhere. It also said champagne would be available, though rationed, for New Year's.

The new regulations announced by Polish authorities Sunday reduce meat rations for everyone but manual laborers, pregnant women and some other people by nearly 30 percent, to 5.5 pounds a month. By comparison, the average American consumes about 15 pounds of meat a month. Butter rations were also cut.

The government also warned that "there will be no possibility of raising food rations in 1982," MTI, the Hungarian news agency, said in a report from Warsaw, according to Associated Press.

The London Times said Sunday that recent visitors to Warsaw hospitals had provided further evidence of a serious shortage of medical supplies, including a lack of antibiotics, which Poland buys from abroad.

In what may have been an effort to convince Western governments that medical aid sent to the Polish military would benefit civilians, Gen. Mieczyslaw Obiedzinski, chief quartermaster of the Polish Army, said in an interview broadcast yesterday by RadioWarsaw that "the military health service has made specialized hospitals and clinics accessible to civilian patients." This year, Obiedzinski said, "about 37,000 civilian patients used military facilities."

Polish authorities said nearly all of the miners who ended their sit-in at the Piast mine in Tychy in southern Poland yesterday were suffering from the effects of their long period underground and were "very weak" but none required hospitalization. A doctor who visited them Saturday had said 80 percent of them needed medical attention, according to Associated Press.

Agence France-Press said the British Broadcasting Corp. monitored a radio ham broadcasting from the Gdansk area, in northern Poland, who said that security forces had flooded out some of the coal mines to force miners to the surface and that soldiers in that area had been shot for refusing to move against workers. He reportedly gave no source for his information.

At one point last week, 3,200 miners were occupying Piast and another major Polish coal mine and thousands more workers were holding sit-ins at shipyards, steel mills and other industrial facilities throughout the country. They were protesting imposition of martial law by Gen. Wojciech Jaruzelski, the premier and Communist Party leader, who said he acted to avert civil war.

The end of the strike came after stern warnings from the government that workers who continued their strike after the Christmas holidays would be severely punished. According to Radio Warsaw, five activists of the independent labor union Solidarity have already been sentenced to up to 3 1/2 years for strike activities.