Wednesday, 17 April 2013 09:38
Crowds applauded as her union jack-draped coffin began its journey from Westminster to St Paul's Cathedral for the funeral.
Four thousand police are on duty but there were no signs of protests.
Some 2,300 people, from 170 countries, are expected to attend. The Queen will be among them.
The hearse was flanked by police outriders as it left Parliament, where her body lay overnight, to travel to St Clement Danes, the RAF church on The Strand.
As it arrived, the coffin was taken into the church, ready to be transferred to a gun carriage to be taken to St Paul's.
A white floral arrangement on top of the funeral bore a card reading: "Beloved mother, always in our hearts."
Prayers were said by St Clement Danes' resident chaplain the Rev David Osborn.
She has been accorded a ceremonial funeral with military honours, one step down from a state funeral.
Speaking on BBC Radio 4's Today programme, Mr Cameron said it would have been seen as extraordinary not to commemorate her life.
"I think it will be quite a sombre event, but it is a fitting tribute to a great prime minister respected around the world, and I think other countries in the world would think Britain had got it completely wrong if we didn't mark this in a proper way,"
Asked about those who wanted to challenge his view of Lady Thatcher, the prime minister said: "Of course people have the right to disagree and take a different view.
"But when you're mourning the passing of an 87-year-old woman who was the first woman prime minister, who served for longer in the job than anyone for 150 years I think it's appropriate to show respect."
He said he had been "partly" inspired, as a young man, to join the Conservative Party by Baroness Thatcher, who he thought was a "very brave woman doing a tough job".
Police were out early on Wednesday with metal crowd control barriers in place along the main route in Westminster.
Baroness Thatcher's coffin will initially travel by hearse from the Palace of Westminster, where it has lain overnight, to the Church of St Clement Danes - the Central Church of the RAF - on the Strand.
It will then be transferred to a gun carriage to be drawn by the King's Troop Royal Horse Artillery, who will lead it in procession to St Paul's.
The processional route will be lined by more than 700 armed services personnel. A gun salute will be fired from the Tower of London every minute while the procession is taking place.
The Dean of St Paul's said the funeral would be "relatively humble" in line with Lady Thatcher's wishes.
The Very Rev Dr David Ison said she had played a large part in planning the funeral over the past six years.
He said the "simple" service would be in contrast to the "pomp and ceremony" surrounding the transit of the coffin.
"Mrs Thatcher wanted something that was very simple and it is not at all triumphalist," he said.
"There is no eulogy, she is only mentioned once or twice in the service. It uses the book of common prayer, which is actually quite austere in places."
As well as the Queen and the Duke of Edinburgh, all 32 members of the current cabinet are attending the service, along with more than 30 members of Lady Thatcher's cabinets from her time as prime minister.
Lady Thatcher, who was Conservative Prime Minister from 1979 until 1990, died on 8 April, following a stroke, at the age of 87.
There will be more than 50 guests associated with the Falkland Islands, including veterans from the 1982 conflict with Argentina, but Argentina's ambassador to London, Alicia Castro, has declined an invitation to attend.
Alan Southern, a former member of the Parachute Regiment who fought in the Falklands War, said: "Lady Thatcher was an absolutely wonderful lady. She loved the armed forces and she did so much for the country, she put the 'great' back in Great Britain."
In total, two current heads of state, 11 serving prime ministers and 17 serving foreign ministers from around the world will attend.
Six police forces from outside London have sent specialist officers to help with escorting foreign dignitaries.
Various roads along the route were closed from 07:30 BST, and Transport for London has advised drivers to avoid Westminster and the City of London. The roads are expected to be re-opened as soon as possible following the funeral.
There were union jacks on display, as well as flags from the US, Canada, Scotland, Poland and the Falkland Islands.
Wednesday's House of Commons sitting has been delayed until 14:30 BST, meaning the cancellation of Prime Minister's Questions, in order to allow MPs to attend - a move approved in a Commons vote on Tuesday.
The Metropolitan Police said it had been contacted by a small number of protesters to say they were planning action on the funeral route in protest at the impact of some of Lady Thatcher's policies while she was in power. Other protests are expected elsewhere.
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BBC home affairs correspondent Danny Shaw said although Monday's bomb attack at the Boston Marathon was not believed to have led to any significant changes in security for the funeral, it was expected that the police and public would be more vigilant.
He said arguably the biggest headache for police was how to respond to any spontaneous protests along the funeral route or close to mourners.
Senior officers acknowledged they had a "difficult" balance to strike between allowing people to express their opinions and maintaining order, he added.
Police have powers to arrest those who use "threatening, abusive or insulting words or behaviour", but Scotland Yard said it was not for the police to "uphold respect".
Lady Thatcher's union jack-draped coffin was placed in the Palace of Westminster's Chapel of St Mary Undercroft overnight on Tuesday.
A short service, led by the Dean of Westminster, was held for members of the family, senior parliamentarians, and staff from Parliament and Downing Street.
The House of Commons speaker's chaplain kept vigil in the chapel through the night.
The chimes of Big Ben will be silenced for the duration of Lady Thatcher's funeral. St Paul's has published a full funeral order of service.