The crash involving multiple vehicles took place between Junction 27 and Junction 26 anti-clockwise. Emergency services including an air ambulance are at the scene.
The accident involved a lorry, minibus and a car.
Essex Police urged motorists to avoid the area.
The force tweeted: "Officers are on the scene of a serious collision on the #M25 near #WalthamAbbey.
"Both carriageways will close shortly.
"If on the motorway, please follow directions from officers on the scene.
"If you can, avoid the area.
"We'll bring you more as soon as we can."
In a later update, police said: "We'd like to thank everyone on the #M25 for their patience.
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"We responded at 6.15pm to reports that a lorry, minibus and a car were involved in a collision btwn J26-27 anti-clockwise. It remains closed.
"If you're travelling before tomorrow's rush hour please take another route."
Pictures show long tailbacks of traffic on the motorway.
In a tweet, National Highways said: "M25 J27 for the #M11 towards J26 for #WalthamAbbey anti-clockwise - the road is closed due to a serious traffic collision - please use alternative routes to avoid the queues."
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A diversion is in place directing motorists to the M11.
However, there are delays on the M11 southbound between Junction 6 and Junction 4 due to multiple incidents.
Motorists caught in the tailbacks on the M25 took to Twitter.
One tweeted: "Not going anywhere soon. Hope people are safe. I don’t mind waiting.
"Grateful to be alive #m25 #crash #pileup #walthamabbey #waltham #patience #police dealing with the matter #thelawofaverages #sodslaw #emergency #services doing a great job."
Another posted a picture of traffic and said: "Major accident on the m25. Stand still traffic for 45 mins."