All traffic is being held on the M61 after the motorway was closed in both directions due to a police incident.
The motorway has been closed between junctions 3 (Kearsley) and 4 (Farnworth) since 10.42am (Wednesday, May 15).
Greater Manchester Police (GMP) said they are responding to concerns for a man's welfare.
A GMP spokesman said: "We had a call at 10.42am to concerns for a male seen on a bridge.
“Police are speaking with the man."
Traffic England is advising that the motorway could remain closed until 2.30pm, but efforts are being made to free trapped traffic.
A spokesman for Highways England said: "We are assisting those held in queues to exit the motorway as soon as possible.
"We can't provide an exact time, but urge people to remain with their vehicles and await Highways England or police to instruct you."
North West Motorway Police have had to ask drivers to remain in their vehicles after a number of people were seen standing on the hard shoulder.
"Please do not turn around until instructed by police/Highways", the force advised.
"Do not use or stand on the hard shoulder and remain with your vehicle as you’ll obstruct responders.”
The incident has led to severe congestion with lengthy tailbacks for westbound traffic to J5 (Westhoughton).
Diversion routes are now in place following the motorway closure.
Southbound motorists should follow the solid black circle as follows:
- Exit the M61 at junction 4 and join the A6 southbound.
- And then join the A5082 northbound.
- Exit the A5082 and take the third exit onto the A666 southbound.
- Rejoin the M61 at junction 2
Northbound motorists should follow the solid black square and follow the same route as the southbound, but in reverse.