The long-running treason trial of Zimbabwe's opposition leader Morgan Tsvangirai neared its end with the defense team saying several key witnesses failed to appear in court.

Defense attorney George Bizos, who defended Nelson Mandela 40 years ago, said three key witnesses failed to turn up, and the state did nothing to try to find them. He said the three men could have provided details of the relationship between the defendant and a Canadian consultant who was the state's star witness.

The witness, Ari Ben Menashe, presented a videotape on which Mr. Tsvangirai allegedly asked for help in having Zimbabwean President Robert Mugabe killed. Mr. Tsvangirai says he was only asking for political advice on how to defeat Mr. Mugabe in the 2002 presidential election.

In arguments in court this week, State Prosecutor Bharat Patel argued that any meeting in which there is discussion of the elimination of the president, combined with other evidence from state witnesses, constitutes what he called - treasonous conduct.

But Mr. Bizos, the defense attorney, cited South African case law, and said mere discussion did not suffice unless it resulted in a conspiracy to take action, or if there was clear evidence of incitement to commit treason during the discussion.

Mr. Bizos said treason charges are open to political abuse, and one needs clear evidence. It cannot be proved on the basis of tenuous innuendo.

Mr. Bizos called the state's key witness, Mr. Ben Menashe, a crook, cheat and a liar. He said the missing witnesses are all Mr. Ben Menashe's associates, and one of them is an alleged criminal, while the other could be a foreign intelligence agent.

Mr. Bizos finishes his final argument Thursday and the state will then be asked for its closing statement, bringing to an end Zimbabwe's longest and most controversial trial.