Two people were taken to hospital this morning after a six-vehicle crash which closed part of the M1.
Five cars and a lorry were involved in the accident, close to the Leicester Forest East service station at about 8am.
A 32-year-old man has been arrested in connection with the crash, which caused severe delays for thousands of motorists. It is not clear which vehicle he was driving.
Both lanes of the M1 were shut and the road reopened at 9.40am - although huge tailbacks were still being reported hours later.
The accident was on the southbound carriageway, between junctions 21 and 22. Firefighters had to cut people out of two of the vehicles - a white Renault Clio and a blue Peugeot.
The air ambulance landed on the motorway and airlifted one woman, thought to be the driver of the Clio, to Coventry and Walsgrave Hospital.
She was described as having "severe, but not life-threatening" injuries. Witnesses described seeing firefighters racing to free the woman, who was unconscious.
Another man, believed to be the Peugeot driver, was taken to Leicester Royal Infirmary. His injuries are said to be not life-threatening.
Three other cars and a lorry were involved in the accident. Several people watched the start of the police investigation into the crash from a service road bridge.
One onlooker, who did not want to be named, said: "You know something serious has happened when you can't hear the traffic.
"I saw the air ambulance over the trees, coming down on to the motorway, and I went to the bridge to have a look. They put the woman from the white Renault in there. She looked unconscious and the paramedics were holding up drips for her.
"They put the guy from the blue Peugeot in an ambulance and another person in another one. Both ambulances drove off really slowly. I everyone is okay."
Another woman said: "Both of the cars at the back look really bad."
Police began to allow vehicles on the southbound carriageway at about 9.40am.
A spokeswoman for area traffic control said tailbacks were expected for last several hours.
She said: "The accident happened at 8.15am. The southbound carriageway was shut and police closed the northbound carriageway a couple of time to allow the air ambulance in and out.
"It's still really bad. There are huge tailbacks up to junction 22, and there are long delays in Leicester as well, because all routes back from the motorway hit the roadworks at Braunstone Way."