Her Majesty, who was accompanied by the Duke of Edinburgh, was dressed for the sombre occasion in a black suit and matching black hat, and pearl necklace and earrings.
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Baroness Thatcher's funeral service took place following the journey of her coffin – draped in a union flag – from Westminster to St Paul's Cathedral on Wednesday morning. The procession was watched by several thousand mourners, some of whom had camped out on Tuesday night.
A small white card nestled in the wreath that adorned the coffin read: "Beloved Mother - always in our hearts."
Along with the Queen and the Duke of Edinburgh, the Prime Minister David Cameron and his wife Samantha, former leader Tony Blair along with his wife Cherie, and Gordon Brown, were in attendance. More than 30 members from Lady Thatcher's cabinets between 1979-1990 are said to have attended.
Some 2,300 people from 170 countries are expected to pay their respects. It is the first funeral of a British prime minister that Her Majesty has attended since that of Winston Churchill in 1965.
Flags were lowered to half-mast above UK Government department buildings in respect of the longest-serving prime minister of the 20th century at 08:00 in the morning.
The formidable leader, famously known as The Iron Lady for her uncompromising style in politics, died at the age of 87 on 8 April, after suffering a stroke.
She has received a ceremonial funeral with full military honours, similar in status to those accorded to the Queen Mother and Princess Diana, and one step down from a state funeral which is generally reserved for kings and queens.
The chimes of Big Ben were silenced during the funeral as a mark of respect and more than 4,000 police officers were on guard to protect the funeral procession.
Baroness Thatcher presided over many of the key moments in British history including the Falklands War, the Miners' Strike and the end of the Cold War.
She did not want a state funeral. "She particularly did not wish to lie in state as she thought that was not appropriate," Lord Bell, Lady Thatcher's spokesman and friend, said. "And she did not want a fly-past as she thought that was a waste of money — somewhat in character you might think."
Baroness Thatcher did request a central role for The Chelsea Pensioners at her funeral. The Army veterans will not only welcome her coffin when it arrives at St Paul's, but will also benefit from donations made in her memory — her family have requested that, instead of flowers.