The line to view the coffin of Queen Elizabeth II is likely to be one of the longest that London has ever seen.
The line already stretches from Parliament, across nearby Lambeth Bridge and along the opposite bank of the River Thames.
Authorities have planned for a 10-mile (16-kilometer) route, with 1,000 marshals, stewards and police officers on hand at any given time to help manage it. An army of other volunteers includes multi-faith pastors and sign-language interpreters.
People are being warned they may have to wait for hours, but they are being given numbered wristbands so they can take food and bathroom breaks without losing their place in line.
When they get to Parliament mourners must pass through airport-style security screening. Prohibited items include liquids, spray paint, knives, fireworks, flowers, candles, stuffed toys and "advertising or marketing messages."