A sentencing hearing for Vakhtang Makhniashvili on assault charges was derailed Tuesday when the father of a missing Toronto teen said he wanted to dismiss his lawyer, adding he had pleaded guilty because he was “under threat and blackmail.”

Standing in the prisoner box, Makhniashvili, 51, said he pleaded guilty in May to three counts of aggravated assault because “I was under pressure.”

Makhniashvili told Justice Rebecca Rutherford he had been sleep deprived while in custody and administered drugs without his consent.

Before the outburst, court heard Makhniashvili had been assessed by a forensic psychiatrist who found that he suffers from a major mental illness.

The hearing related to two stabbing incidents in 2010 — one involving a former neighbour and the other a married couple who once bailed him out.

Makhniashvili first made headlines when his daughter Mariam vanished outside her high school in September 2009, which launched an unprecedented search by Toronto police for a missing person. Her whereabouts is still unknown.

Defence lawyer Calvin Barry has said her disappearance caused Makhniashvili tremendous strain, which may explain his subsequent violent outbursts.

On Tuesday, as Makhniashvili announced he wanted to fire his lawyer, his wife Lela Tabidze looked on in disbelief.

“I can’t lie . . . I signed it by pressure. My plea was under threat and blackmail,” Makhniashvili told the court.

Proceedings were twice interrupted so Makhniashvili could speak with his lawyer, but it remained unclear if his guilty plea had been voluntary. The hearing was adjourned until Thursday, when Makhniashvili will decide if he wants to continue or hire a different lawyer.

Outside court, Barry said Makhniashvili has been “under a lot of stress” and on medication.

According to an agreed statement of facts, on May 7, 2010, Makhniashvili stabbed neighbour Sean Ure in the abdomen, believing he was involved in Mariam’s disappearance.

Private investigators David and Delores Langer — who had never met Makhniashvili — posted $50,000 bail for his release on charges stemming from the attack on Ure.

When he grew suspicious of their motives, they withdrew their bail. His wife became his new surety. He had no contact with the Langers until November, after his son George, 18, ran away for a night.

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In a column, the Toronto Star’s Rosie DiManno questioned whether the Makhniashvili home was the best place for George. The next day, on Nov. 4, Makhniashvili went to the Langers’ home and stabbed David in the abdomen and slashed Delores, believing they were responsible for the column.

The defence will request a sentence of two years less a day, plus time spent in pre-trial custody, to be served in a provincial reformatory. The Crown will seek a penitentiary sentence of several years.