The accident closed the motorway in both directions between the M25 and junction 2A eastbound and between J3 and the M25 westbound, resulting in miles of queues.
By 4pm, Kent Police said one lane on the coastbound carriageway had opened to traffic, while the London-bound carriageway was expected to stay closed until 10pm.
Multi-vehicle crash
Police were called to the scene at 9.30am after the crash involving two HGVs, two cars and a box van, happened on the London-bound carriageway, near Kemsing.
Seven people were taken to hospital and emergency teams remain at the scene.
Kent Police have appealed for anyone who witnessed the collision, or the events leading up to it, to contact the Serious Collision Investigation Unit on 01622 798 538.
Traffic on the Londonbound carriageway is being turned round and redirected to join the M20 at Wrotham Heath.
TRAFFIC: Motorists advised to avoid #M26 as both directions could remain closed for several hours after serious collision.