LONDON (Reuters) -Mourners will be able to file past the coffin of Britain’s late Queen Elizabeth in a round-the-clock lying-in-state from Wednesday evening to early on the day of her funeral, with authorities preparing for a huge turnout.

Hundreds of thousands of people are expected to pay their respects to Britain’s longest-reigning monarch ahead of her state funeral on Sept. 19, which will be attended by world leaders.

Rail operators said they expected “unprecedented travel demand” within London and advised passengers to consider walking to their final destinations.

Britain’s culture ministry said members of the public would be able to file past the coffin in parliament’s Westminster Hall for 24 hours a day from 5.00 p.m. (1600 GMT) on Wednesday Sept. 14 until 6.30 a.m. on Sept. 19.

“Those wishing to attend will be required to queue for many hours, possibly overnight,” the ministry said. “Large crowds are expected and people are encouraged to check ahead, plan accordingly and be prepared for long wait times.”

Lying-in-state is a rare honour in Britain, previously accorded on only nine other occasions and mostly for royal family members, including Queen Elizabeth’s mother in 2002 when an estimated 200,000 paid their respects.

Queen Elizabeth’s closed coffin will rest on a catafalque — a raised, decorated platform used to support it — inside Westminster Hall.

Each corner of the platform will be guarded around the clock by units from the Sovereign’s Bodyguard, Foot Guards or the Household Cavalry Mounted Regiment.

Airport-style security checks will take place at Westminster Hall, the oldest of the buildings that make up Britain’s parliamentary estate, and tight restrictions will be in force with only small bags permitted.

The lying-in-state will be preceded by a ceremonial procession through central London to carry the queen’s coffin from Buckingham Palace to Westminster Hall on Wednesday.

Elizabeth died on Sept. 8 at Balmoral Castle in the Scottish Highlands and her coffin was driven to Edinburgh on Sunday. It is due to be flown to London on Tuesday.