Lady Thatcher's union jack-draped coffin was placed in a chapel in the Palace of Westminster on Tuesday.
A security operation involving more than 4,000 police officers is getting under way for the funeral of former UK Prime Minister Baroness Thatcher.
Lady Thatcher's coffin will travel from Westminster and be taken in procession through central London for the funeral at St Paul's Cathedral at 11:00 BST.
Some 2,300 people, representing 170 countries, are expected to attend. The Queen will be among them.
Scotland Yard said it was expecting some protests along the funeral route.
Lady Thatcher, who was Conservative Prime Minister from 1979 until 1990, died on 8 April, following a stroke, at the age of 87.
She has been accorded a ceremonial funeral with military honours, one step down from a state funeral.
Her 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 will attend the service, along with more than 30 members of Lady Thatcher's cabinets from her time as prime minister.
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.
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 will be 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.
Hours before the funeral, people began to gather outside the cathedral. Estimates by BBC reporters ranged from about 70 to 300 positioned behind barriers before 07:00 BST.
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.
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.