WARSAW, Poland — Polish groups campaigning for a bill on returning real estate seized, stolen or sold under duress decades ago have invited American politicians to see firsthand the restitution irregularities the groups say occur without proper regulation.

Last week, 59 U.S. senators wrote Poland's prime minister to urge the passage of legislation that would compensate the heirs of people who lost property during World War II and later under communism.

Poland is central Europe's only country without such a law. Individual cases are handled by courts, but activists say criminal activity has caused many plots and houses to be given to the wrong people.

Polish activists and lawmakers who are combatting what they call a "restitution mafia" invited the senators on Wednesday to tour sites in Warsaw that were transferred to people unrelated to the original owners.