Part of the M25 is shut for the first scheduled daytime closure of all lanes since opening in 1986.
There are fears thousands of motorists could get stuck in gridlocked traffic after the closure of a section of the M25 in Surrey for the entire weekend.
A five-mile stretch along both carriageways between junctions 10 and 11 was closed at 21:00 GMT and will remain shut until 06:00 on Monday.
National Highways says about 270,000 drivers use the stretch on a weekday.
Project lead Jonathan Wade said: “Drivers should only use the M25 if their journey is absolutely necessary.”
It is the first time there has been a scheduled daytime closure of all lanes on the M25 since it opened in 1986.
The closure is to enable the demolition of a bridge and the installation of a new gantry as part of the junction 10 improvement scheme, which is due to be completed in mid-2025.
National Highways said modelling carried out in the development of the plans indicated vehicles would face delays of up to five hours without mitigation measures, such as urging drivers to stay away.
The agency said the carriageway between junctions nine and 11 typically carried between 4,000 and 6,000 vehicles per hour in each direction between 10:00 and 21:00 at the weekend.
Mr Wade told drivers to “avoid the area entirely, change their plans or take the train”.
He also advised motorists to avoid “simply following what a sat-nav says” and to follow signs in the area.
Some school sports fixtures scheduled for this weekend in Surrey have been postponed because of the closure.
Meanwhile, St Peter’s Hospital in Chertsey, Surrey, warned patients to “only visit if necessary”, and the Royal Horticultural Society says its garden in Wisley “will try to remain open”.
Travellers affected may include people going to Gatwick and Heathrow Airports and the Channel ports, as well as concert-goers, sports fans and holidaymakers travelling to London and surrounding areas, National Highways said.
Tahir Aziz, of Woking Borough Council, said: “We’ve never experienced something like this before.”
Malcolm Cressey, of Runnymede Borough Council, said he expected “gridlock” in the area.
The closure will be the first of five between now and September as part of a £317m upgrade project.