STORY: Her coffin was brought to Westminster Hall earlier on Wednesday from Buckingham Palace.
People started waiting in line late on Tuesday, sleeping on the streets in the rain, to be one of the first to file past the coffin, and there was already a queue over two miles long.
“She meant everything because she had been Queen all of our lives,” said Ruth Reed, who came to give respect to the queen.
The government has warned the queue could eventually stretch for up to 10 miles (16 kilometres) along the southern bank of the River Thames, winding past landmarks such as the giant London Eye Ferris wheel and a reconstruction of Shakespeare's Globe theater.
Elizabeth's coffin was flown back London late on Tuesday from Scotland, where it had been since her death at her Scottish summer holiday home Balmoral Castle.
As many as 750,000 mourners are expected to walk through Westminster Hall to pay their final respects.