Poland's last communist leader, General Wojciech Jaruzelski, is due in court charged with ordering the shooting deaths of 44 shipyard workers.
The trial is part of Poland's long-running effort to call the old communist regime to account for its crimes.
It has been delayed for years by repeated failure of Jaruzelski or his co-defendants to appear in court on grounds of ill health.
The retired general, 78 in July, maintains he is innocent. He and nine other defendants each could get 25 years if convicted.
Jaruzelski is charged with ordering the military to shoot at workers protesting price rises while he was defence minister. Forty-four workers were killed in the Baltic coast cities of Gdynia, Gdansk, Szczecin and Elblag on December 17, 1970. More than 1,000 were wounded, 200 seriously.
An investigation by the communist regime was dropped, and there was never a trial.
Jaruzelski was Poland's leader from 1981, when he imposed a martial law to crack down on the anti-communist Solidarity movement, until 1989, the year communist rule finally was toppled.
The court is expected to heed advice from doctors that hearings be limited to three or four hours and that Jaruzelski's condition be considered before each session in the trial, which could last more than a year.
The general repeatedly has denied the charges, saying that Wladyslaw Gomulka, the country and party leader at the time of the shootings, disliked him and kept him out of key decisions. Other defendants say they were not present or gave no orders to shoot.