SINGAPORE - Left alone with his elderly father for three hours in their Bedok North flat, an "unstable" unemployed man allegedly attacked the 75-year-old, who was later found lying in a pool of blood.

As paramedics attended to the victim, the man, Tan Kok Meng, covered in dried blood, suddenly got up from the sofa where he was sitting in a daze.

Ignoring the pleas of the paramedics, he throttled his father while mumbling "I want him to die" in Mandarin for a minute or so, then went back to sit on the sofa, allowing the paramedics to resume attending to the victim.

The victim, Mr Tan Ah Hin, stopped breathing in the ambulance and was pronounced dead at Changi General Hospital.

The account emerged in the High Court on Tuesday (Aug 11) as Tan went on trial on a charge of murder for the death of his father on Nov 13, 2015. Tan has put on weight since he was charged in 2015 and now has a head of grey hair.

The prosecution has accused him of inflicting multiple blows on the victim's face and strangling him.

Deputy Public Prosecutor Daphne Lim told the court that there is some evidence showing that Tan would qualify for the defence of unsoundness of mind.

Dr Subhash Gupta from the Institute of Mental Health diagnosed him with schizophrenia.

Dr Cheow Enquan, also from the IMH, said that in addition to suffering from schizophrenia, Tan was also in a state of acute drug intoxication, as he had taken methamphetamine at the time.

The DPP sought an order for Tan to be detained at the President's pleasure.

This means that if the court finds Tan committed the alleged acts, he will be acquitted due to his mental incapacity but will be confined in a psychiatric institution, prison or some other place of safe custody.

There is no minimum period of detention, and the mental state of the detainee is reviewed by psychiatrists regularly until he is found to be suitable for discharge.

Tan is defended by Mr Favian Kang and Mr Nichol Yeo.

The DPP told the court that on Nov 13, 2015, the younger Tan was not his usual self.

His mother, Madam Toh Meow Siang, noted that he had been looking dazed for the past two days and had been pacing up and down the flat.

To keep him from leaving the flat, Madam Toh kept the house keys from him. Before she left the flat at about 2.30pm, she told her husband not to allow their son to leave the flat.

About three hours later, she returned to find her husband breathing heavily, lying face up in a pool of blood under his head.

She asked Tan, who was seated on the sofa, what happened but he did not respond.

Madam Toh left the flat to get help from the neighbours, and when she returned, she shouted at the accused, asking why he killed his father.

Tan then walked over to the older man, sat on his abdomen and placed his hands on his father's chest, just below the throat, before Madam Toh pulled him away.

The paramedics arrived soon after.

Ms Zaneta Lee testified that while she was inserting a device to clear the victim's airway, Tan suddenly started choking his father.

She said she shouted at Tan to stop but he ignored her. "We did not try to pull the accused away, as we were worried for our safety. We did not know if he had a weapon," she said.

Police officer arrived shortly after and arrested Tan.

He told officers that he used his bare hands to attack his father, that they had a strained relationship and that the older man had called him "good for nothing".

An autopsy found the older Tan died from strangulation and blood aspiration. Extensive bruises were also found on his face, chest and arms.

Forensic pathologist Associate Professor Teo Eng Swee said the main source of the blood was likely from a deep cut on the tongue, which was consistent with an account of the victim being punched in the face.