Dr Andrew Wakefield, the doctor who first suggested a link between the MMR vaccine and autism, is facing a string of charges of serious professional misconduct.
Andrew Wakefield briefly met supporters before the hearing
It is the latest development in a long and controversial saga over the vaccine.
The charges against Dr Wakefield relate to research published in The Lancet medical journal in February 1998 suggesting there could be a link between the triple jab, bowel disease and autism.
Dr Wakefield and his former colleagues, Professors John Walker-Smith and Simon Murch, are said to have acted unethically and dishonestly towards the Lancet. They all deny the allegations in the case.
The charges against Dr Wakefield include accusations that 11 children were subjected to invasive tests such as colonoscopies and lumbar punctures that were contrary to their best clinical interests.
Dr Wakefield is accused of not having the "requisite paediatric qualifications" or of seeking the right approval for the tests.
He is also accused of acting "dishonestly and irresponsibly" in failing to disclose how he recruited patients to the study. The hearing against him was told Dr Wakefield paid children £5 for their blood samples at his son's birthday party.
Another of the key allegations against Dr Wakefield is that he was being paid at the time for advising solicitors on legal action by parents who believed their children had been harmed by MMR.
He is also accused of abusing his position of trust and bringing the medical profession into disrepute.
Dr Wakefield and Professors Walker-Smith and Murch are appearing before the General Medical Council (GMC) Fitness to Practise Panel in central London.
They face being struck off in a case that could last into the autumn.
Millions of doses of the MMR vaccine have been used since it was introduced in the UK in 1988.
The furore that followed Dr Wakefield's suggestion that the MMR vaccine was unsafe led to fewer parents immunising their children and a row over whether the then prime minister, Tony Blair, had vaccinated his son, Leo.
Mumps, measles and rubella are all serious diseases and many doctors are concerned that a drop in vaccination levels could leave many children at risk.
Dr Wakefield's supporters gathered outside the hearing
The UK government and the vast majority of scientists insist the three-in-one jab is safe.
The Department of Health says there is "no reliable scientific evidence" showing any link between the MMR vaccine and autism. It insists that the three-in-one jab is the safest way to protect children.
And there have been many studies examining the safety of MMR since 1998. All have concluded that the MMR jab is safe.
Last year The Lancet, which published the controversial MMR paper in the first place, publicly announced it should never have printed it.
But for a small group of parents who believe their children have suffered as a result of the MMR vaccine, Dr Wakefield is a hero.
Dozens travelled to London to noisily show their support for Dr Wakefield and his colleagues.
Waving banners and brandishing pictures of their children they denounced the hearing as part of a "witch-hunt" being carried out by the medical establishment.
They said the charges had been trumped up.
Around 100 campaigners, including some from campaign groups Jabs and Cry Shame, gathered in the street outside the building where the hearing was being held.
Dozens of placards were placed on railings with cards pinned to them bearing messages of support from the families of autistic children.
As part of a "One Flower One Child" initiative hundreds of multi-coloured carnations had been placed in front of the placards in several pots.
An array of banners and posters declared support for the trio facing the panel inside. One read: "We're with Wakefield, Munch and Walker-Smith. Stop the witch hunt".
Others read: "Clear the MMR III" and "Normal before, autistic after. Why?"
Among the crowd was Celia Forest, whose 13-year-old son Adam took part in Dr Wakefield's study after falling ill three months after having the MMR jab.
She said he became withdrawn, experienced dramatic body temperature changes and had severe gut pain. He was eventually diagnosed with autism.
"I think this witch-hunt is a disgrace. It is being conducted by the British medical establishment.
"Because of this many GPs are too scared to acknowledge the fact that a child has a serious bowel disorder and do not make proper treatment available for them.
"They are scared of being victimised like these doctors. It's all about the drug companies and the money. Vaccines are big business. There is a lot of anger here."
Also among the demonstrators were Sara and Chris Fisher, whose 18-month-old son George died last year 10 days after receiving the MMR jab.
Sara said: "There's no doubt in my mind that the jab was responsible. He was a healthy boy, big and strong.
"They should do more research into MMR rather than spend time persecuting these doctors."
Experts said the hearing was likely to be long and complex. But, whatever the outcome, Dr Wakefield's supporters are unlikely to stop believing in him.