The incident occurred on the northbound carriageway between junctions 6 (Horwich) and 8 (Chorley) at approximately 12.30pm on Friday (April 8).
A car transporter overturned, causing one of the vehicles being carried by the lorry to land in the southbound carriageway.
The incident resulted in the closure of the northbound carriageway while a “complex recovery and clean up” operation took place.
Two out of three lanes were also closed on the southbound carriageway while emergency services attended.
“Please avoid the area and use alternative routes where possible,” a spokesperson for Lancashire Police said.
“Emergency services are currently on scene.”
National Highways warned the closure was likely to remain in place for several hours.
Road users were advised to follow the solid triangle diversion symbol on road signs:
- Leave the M61 northbound at junction 6
- At the roundabout take the A6027 westbound
- At the next roundabout take the A6 northbound
- At the A6/A674 roundabout take the A674 eastbound
- Re-join the M61 at junction 8
A spokesman for National Highways said: “Congestion on the diversion route is already very heavy so drivers should delay journeys or seek alternative routes.
“Traffic officers are at the scene helping to manage traffic.
"Staff at the North West Regional Operations Centre (NWROC) near Warrington and National Traffic Operations Centre (NTOC) at Birmingham have set electronic signs across the motorway network to warn drivers of the carriageway closure.”