Emergency services were called to East India Harbour off the coast of Greenock after the boat overturned.
A major rescue operation has been launched after a tug capsized off Greenock in the west of Scotland.
Emergency crews were called to East India Harbour on Friday afternoon after the boat overturned.
Images from the scene showed rescue teams in inflatables and a police boat surrounding the capsized tug while a helicopter hovered overhead.
It is not known how many people were on board, or if there have been any casualties.
Rescuers could be seen climbing onto the overturned hull of the boat before it sank.
Eyewitnesses told BBC Scotland they had seen the tug escorting the Hebridean Princess cruise ship into the harbour at about 15:30 when it was apparently pulled over.
Daniel McBride said the tug had capsized “pretty instantaneously”.
He added: “At that point I contacted the coastguard and was asked to go and keep eyes, so I parked up and watched.
“Within 12 minutes the first coastguard vessel came. At that point the boat was still capsized with a hull visible in the water.
“I witnessed them bashing on the hull, I guess trying to see if there was any signs inside. Unfortunately then the boat went down a short time afterwards.”
He said there were a number of boats in the area, as well as a helicopter, but he had not seen anyone being pulled out.
Police have cordoned off the area around the harbour.
A coastguard spokesman said: “Coastguard Rescue Teams from Helensburgh and Greenock, a lifeboat from Helensburgh and the Coastguard helicopter from Prestwick were sent to assist and are searching the area.
“Police Scotland is also in attendance.”