Harris, 84, is the biggest name to go on trial since British police launched a major investigation sparked by revelations that the late BBC TV host Jimmy Savile was a prolific child sex abuser, leading to the arrest of more than a dozen ageing celebrities.
Harris is known by millions in Britain and Australia for pop chart hits such as "Two Little Boys" and "Tie Me Kangaroo Down, Sport."
A keen artist, he painted Queen Elizabeth's portrait in 2005, was honoured by her for his services to entertainment and charity and performed at her 2012 Diamond Jubilee concert.
Harris's daughter, Bindi, and his wife, Alwen Hughes, were at his side as he arrived at Southwark Crown Court.
The white-bearded, bespectacled entertainer sat silently in the dock as he was accused of 12 counts of indecent assault against four girls between 1968 and 1986, the youngest of whom was aged just seven or eight at the time of the alleged offence.
He denies the charges.
On the opening day of what is expected to be a six-week trial, the 12-member jury of six men and six women heard that Harris had a reputation at an Australian TV channel for his inappropriate behaviour.
"He was known as the octopus because of the way he would put his hands all over women," said prosecutor Sasha Wass. "He took advantage of his fame and popularity."
JEKYLL AND HYDE
Wass said Harris was a Jekyll and Hyde figure, whose "dark side" was not known to colleagues and companions for decades.
He developed a consistent approach in gaining the trust of his young victims before abusing them, the jury heard.
"You will see a pattern during the case, of Mr Harris approaching girls in a purely friendly way and then once he is in close physical contact with them, taking advantage of the situation in order to indecently assault them," Wass said.
The court heard that Harris wrote a letter of confession to the father of one of his victims in which he accepted that he had had a sexual relationship with the girl but did not say it had occurred whilst she was under-age.
Harris abused the girl from the age of 13, in numerous locations in Britain and elsewhere, the court was told. His status meant she did not scream, shout or complain at the time, only confiding in her brother in 1996.
Wass said he abused his victims in numerous locations such as Hawaii and Australia, but many incidents took place before a law change in Britain meant offences allegedly committed abroad could be tried in the UK.
The jury will not be asked to decide on those alleged offences.