Seven British aid workers are being held captive on a ship in the Libyan sea today in a dispute with the captain over money.
Volunteers on the Road to Hope convoy, an independent humanitarian group bringing aid to Gaza, were taken out to sea after the row with the Greek captain over a payment.
The rear of the ship is only partly closed, and Road to Hope workers are "extremely concerned" for the safety of those onboard.
Ellie Merton, London liaison for the convoy, said: "We are astonished that this has happened - shock isn't the word.
"As far as I know, the boat owner became really aggressive over a payment for something, tried to shut the back end of the boat and moved away from the pier really fast.
"The boat was still attached to the quayside and as he drove he yanked the ropes off, there were bits of concrete flying everywhere.
"We have lost telephone contact with those who are still on the ship against their will - without any paperwork, passports or authority to leave Libya, possessions or, when we last heard, food or water.
"We're extremely concerned - the boat's not seaworthy and the captain has shown he is willing to risk life and limb of himself, his crew and the aid workers."
The ship is now in international waters, where it has been surrounded by Libyan navy ships and two fighter jets.
Officials have been negotiating with the captain since this morning.
As well as the seven British volunteers, two Irish, one Algerian and three Libyan officials are on the ship, all aged between 24 and 60 years old.
Among the members of the convoy are Ken O'Keefe, a former US marine, and Laura Stuart, a mother of three from Barnet, north London, both survivors of the Mavi Marmara attack in May this year.
Convoy leader Kieran Turner, a humanitarian aid worker from Liverpool, is also one of those held captive at sea.
A spokesman for the Foreign Office said: "We are aware of the incident at Derna Port in Libya, and have been in close contact with the convoy organisers.
"Our Embassy in Tripoli has been urging the Libyan authorities to resolve the situation rapidly and ensure that those caught on the ship are returned to safety."