British businessman Shrien Dewani has formally pleaded not guilty to murdering his wife on their honeymoon in South Africa in 2010.
Mr Dewani, from Bristol, is accused of hiring a hitman to kill his wife Anni.
The couple were held at gunpoint while being driven in a taxi through Gugulethu township near Cape Town.
Mr Dewani, 34, told Western Cape Crown Court in a written statement that his "whole world came crashing down" when his wife was found killed.
The businessman also revealed to the court that he is bisexual.
Mr Dewani faces five charges including murder and lying about the circumstances of Swedish national Anni's death.
Prosecutors argue that Mr Dewani conspired with Cape Town residents Zola Tongo, Mziwamadoda Qwabe and Xolile Mngeni to kill his wife.
The court heard forensic evidence that the fatal shot was delivered "at close range", with a suggestion that Mrs Dewani might have been grabbing on to "someone or something" at the time she died.
A video taken shortly after her bloodied body was found was also shown to the court.
Through defence lawyer Francois Van Zyl, Mr Dewani said he had "had sexual interaction with both males and females".
"I consider myself to be bisexual," the court was told.
"My sexual interactions with males were mostly physical experiences or email chats with people I met online or in clubs, including prostitutes," Mr Dewani's witness statement said.
Mr Dewani said he had abnormally low levels of hormones, rendering his chances of having children slim. He said he discussed this with Anni, whom he began dating in summer 2009.
The court also heard of a volatile relationship between the newlyweds, including an "angry disagreement" in May 2010.
A letter sent later from Mr Dewani said "I really do love you. Want to be with you forever."
But Anni wanted to call off the wedding, claiming Mr Dewani was "too controlling", the court heard.
"We really frustrated each other, we were in love," he said through his lawyer.
At the scene - Karen Allen, Africa correspondent
Shrien Dewani stood upright and for the most part composed on the first day of his trial.
It is a trial that many believed would never be held in South Africa after a protracted extradition battle and concerns about Mr Dewani's mental health.
Defence lawyer Francois van Zyl revealed that Mr Dewani had said he was "bisexual", a fact that may serve to neutralise some of the fevered press speculation of recent months and police statements given by a gay escort who claims he spent time with Mr Dewani.
The coming days are likely to see a detailed public examination of the couple's relationship.
But in a statement read out in court Mr Dewani said he had been "instantly attracted" to Anni when they first met and he sobbed quietly when a "love letter" he wrote to her after an argument, was read out in court.
The pair married later in 2010 and travelled to South Africa for their honeymoon. According to Mr Dewani, he bought flexible tickets for the trip.
It was when the couple arrived in Cape Town that Mr Dewani met taxi driver Zola Tonga, who has already admitted his part in Anni's murder.
Tonga told Mr Dewani he was an "executive tour guide", and was asked to hire a helicopter as a surprise for Anni.
The driver, according to Mr Dewani, also helped to change £5,000 in order to get "a good market rate".
On the night of Anni's death Mr Dewani was carrying "a large amount of cash", and Tongo had texted him to ask if he still had money for the helicopter ride.
Later, when they were in the taxi, Tongo pulled off the motorway and the car was stopped, the court heard.
"The next thing I recall is somebody next to me, who told me to lie down. The person had a gun in his hand. He was waving the gun in the air," Mr Dewani's statement said.
"We were both terrified and immediately complied with his demands. I was lying half on top of Anni. Another person was behind the steering wheel. I do not know where Tongo was at that stage."
Then, one of the gang asked Mr Dewani to get out of the car. When he refused, the court heard, a gun was held at his head before he got out of the car.
He then tried to find help before being taken back to his hotel by police.
Mr Dewani said he later met Tongo, whom he felt "sorry" for, and had planned to give him some money.
The case was adjourned until Wednesday.
Extracts of a letter from Shrien Dewani to Anni, dated May 2010
I realise we are very different but I have always believed in a relationship you can work through those differences. When we first met I immediately liked you ... And no not just because you are pretty ... but because you made me laugh.
I have always wanted a girl that I can be friends with. One that understands me - and I know that that is not easy. I know that I am so focussed that some people think I am intense. I am focussed on achieving things in life.
I really do love you, and hence I don't want you to be unhappy. I want to be with you forever but not if that makes you unhappy ... that I could not bear ... I really hope we work this out.
I am really sorry that I have made you feel like this. You are so precious to me - I know I don't always show it. I often find it difficult to show how much you mean to me ... But please do not think this is because I don't love you.
Speak to you later
South African Xolile Mngeni was convicted of premeditated murder for shooting Mrs Dewani and jailed for life.
Prosecutors claimed he was hired to carry out the killing.
Zola Tongo, was jailed for 18 years after he admitted his part in the killing and another accomplice, Mziwamadoda Qwabe, also pleaded guilty to murder and was jailed for 25 years.
The start of proceedings in a South African court follows a near four-year legal battle to bring Mr Dewani to trial.
He was arrested on suspicion of conspiracy to murder in December 2010 at the request of the South African authorities who said they would initiate extradition procedures.
Mr Dewani said he would not consent to being extradited.
Lawyers argued Mr Dewani was not fit to stand trial and that he would be mistreated in a South African prison.
He was detained in a hospital having been diagnosed with post-traumatic stress disorder.
Later, when Mr Dewani's health improved, a judge ruled he could be extradited and, despite winning the right to appeal to the Supreme Court, he was put on a plane to Cape Town in April.
Since then he has been held at the Valkenberg hospital unit awaiting trial.