The rococo mansion at 25 Mokotowska St. is a rare jewel in the center of this city which saw its architectural heritage laid waste at the end of World War II and replaced by gray Stalinist blocks. But as much as 25 Mokotowska is a reminder of lost Warsaw, it also carries other historical baggage: To whom does it rightly belong?

The Broniewskis, a leading Polish family of landed gentry and industrialists, fled their lands, factories and homes, including the mansion on Mokotowska, in 1939 as the Nazis advanced toward Warsaw. They survived the war, first in Romania and then hiding in the French Alps.

When they returned in 1947, they discovered that the Communist Party had occupied their property. In 1976, the party turned 25 Mokotowska over to the International Mathematical Center, a branch of the Polish Academy of Sciences, which fiercely defends its right to stay there.

But Warsaw resident Maria Broniewski, 70, whose married name is Gordon-Smith, wants the mansion back--and by asserting her claim she has waded into one of the most emotional, complex and long-running issues in democratic Poland: How can the country compensate those Polish Roman Catholics and Jews whose private property was seized during World War II by the Nazis or afterward by the Communists?

"We just want our family home back," said Gordon-Smith, whose family also lost factories and a 2,000-acre estate near Lublin, which is now divided into small farms. She said her family has no wish to displace small farmers and will forfeit the land. But as for the house, she says firmly, "no."

"Mokotowska 25 was stolen," she said. "It is very precious to us and it is ours, legally and morally."

Poland, alone among Central European countries, has passed no law on the return of private property. Restitution bills failed in 1991 and 1993 because the original owners, and their supporters in Parliament, thought too little was being given back, and those opposed to restitution, particularly the Communist successor party, thought the bills gave away too much. The government is about to try again and a draft bill on restitution will have its first reading in Parliament this week.

The number of claimants and the range of property in dispute in Poland are staggering. The government estimates that it faces 170,000 claims from former owners or their descendants--a number that the Polish Union of Property Owners says underestimates the actual number by half. The total value of the potential claims runs to tens of billions of dollars.

No corner of Poland is untouched by the issue. All property in the city of Warsaw, for instance, was nationalized after the war. Farmland, forests, lakes, palaces, mansions, factories and family residences across the country are subject to claims. And some of the disputed property has been sold by the state since the collapse of communism in 1989, bringing a third aggrieved party--the new owner.

Some of Poland's signature properties are in dispute. Twenty-six Polish families, for instance, are demanding compensation because they were driven from their homes in central Warsaw in the 1950s so Soviet leader Joseph Stalin could bestow the Palace of Culture, a gray and much-loathed skyscraper, on the Polish people. A Polish-Canadian family says the Communists gave their central Warsaw palace illegally to the U.S. government, which tore it down to build a new embassy. And a New Yorker is claiming a building in Wadowice--the home where Pope John Paul II grew up outside Krakow and which is now a museum.

Poland is under increasing pressure to sort out the property claims. The European Union has tied the issue to Poland's application for EU membership. American and British Jews have filed class action lawsuits in New York and Chicago seeking the return of seized property. On Wednesday, a Chicago federal judge dismissed one of the lawsuits, saying the case does not belong in a U.S. court and accepting the argument of Poland's lawyers that a sovereign nation cannot be sued in the courts of another country.

The New York suit particularly inflamed opinion here and, if anything, hardened attitudes against returning property. The suit alleged that the postwar seizure of Jewish property was driven by Polish antisemitism and was "part of a systematic scheme to wipe out all traces of the Jewish race." The suit even used the Nazi word, Judenrein, or "free of Jews." It alleged that the "ethnic and racial cleansing" of Jews in Poland continued after the war to the present.

"To write that the Polish governments--even the Communist ones to which I was most certainly opposed--'propagated the Nazi plan of Judenrein,' one has to be a scoundrel without a conscience," wrote Adam Michnik, a leading former dissident and editor of the newspaper Gazeta Wyborcza, who is Jewish.

The New York lawyers retracted some--but not all--of their accusations, but the furor over the suit has complicated the already difficult task of passing a bill. The proposed law would compensate those who lost property with long-term government bonds for 50 percent of the current value of the property. The bill would only compensate those who were Polish citizens at the time their property was taken--a provision that could exclude some American Jews who fled Poland and excludes all Germans who lost property in what is now western Poland and was previously part of Germany. As in 1991 and 1992, however, the bill seems to fall between competing views of what is just.

"You don't reward theft by saying you can keep 50 percent of what you stole," said Miroslaw Szypowski, president of the Union of Property Owners. Szypowski said all property, or its full current value, should be returned.

Others think that is too much for Poland to bear. "Re-privatization is an ambush," wrote the novelist Andrzej Szczypiorski. "And what's worse, no one will be satisfied. . . . One should think, who really used the facilities? Was it the Communist apparatus or orphans, patients, students and average citizens?"

In other words, the disputed property has gradually accreted to the nation. In fact, no property, such as parks and palaces, deemed important to Polish culture is covered by the bill.

That is exactly the argument employed by the International Mathematical Center.

"To talk about a right after 50 years is very problematic," said Bogdan Bojarski, a professor of mathematics and the center's director. "The tragedy of war fell on all of us and there is no moral justice saying that all the people should now pay back to a small group."

Showing a violation of Communist law--the only existing mechanism to retrieve property--Maria Gordon-Smith and her brother Bogdan had the nationalization of the family home annulled in 1993. But the mathematicians have filed appeal after appeal, each time claiming new grounds why they should retain occupancy. They cannot be evicted while the appeals proceed.

CAPTION: Maria Broniewski wants to get her family home back, but the International Mathematical Center, which now occupies it, is fighting to stay.

CAPTION: Mieczyslaw Broniewski stands in front of the family home in the 1930s. The family fled as Nazis approached Warsaw in 1939.