The volunteers were planning to head to Gaza when the dispute happened. British aid workers, who were allegedly taken captive on-board a ship on Thursday, say they are being held at gunpoint by Greek commandos.
The captain of Strofades IV took the volunteers, who were on the Road to Hope convey, to his native Greece after leaving Derna Harbour in Libya.
Tauqir Sharif, from London, who is on-board the ship said soldiers in Greece were treating the group like “terrorists”.
The Foreign Office is investigating.
The British Embassy in Athens has also told the BBC it was in contact with the ship’s owners and the Greek authorities and was trying to ensure a safe resolution.
Mr Sharif, from Walthamstow, told the BBC the ship’s captain, who has not been identified, had accused the charity workers of being terrorists.
Ellie Merton, the London liaison for the group also told the BBC: “The guys have just arrived at Piraeus Harbour – the main port in Athens.
“They are being treated like terror suspects by the Greek commandos and we are extremely concerned.
“I have been sent text messages from those on-board and I am trying to get hold of the Foreign Office about this situation.”
During a terse mobile phone call from the deck of the Strofades IV cargo ship, charity worker Tox (Tauqir) Sharif told me that Greek commandos boarded the vessel as it approached the port of Piraeus.
He said seven aid workers from Britain and two from Ireland were being held.
“Six of us are Muslims and have beards, it’s not looking good,” he said, adding that the captain had called them terrorists.
According to Mr Sharif, Libyan naval vessels and aircraft set out in pursuit but gave up when the Strofades IV entered international waters.
Mr Sharif said a Greek warship and coastguard vessel began shadowing the cargo ship as it approached Piraeus.
The other British nationals on-board include Mustapha El-Guerbouzi Raheal Parveez, Yunus Malik, Nagib Elgarib Elbarrami, all from London, and Kieran Turner, from Liverpool.
Khalid Mohamed Omar Ali, whose hometown has not yet been reported, is also from the UK.
Algerian Aziz Mekkati, and David Callender and Kenneth O’Keefe from Ireland are also on-board.
There are also seven Libyans on the boat. They are said to include customs inspectors, border police and the harbour manager for Derna Port in Libya.
Members of the group were also reported to be updating their Facebook profiles with updates, revealing that the group had been bodysearched at around 0900 GMT.
Mr Sharif, who is also known as Tox, was also on the Gaza-bound aid ship Mavi Marmara which tried to breach an Israeli naval blockade in May this year.
In a statement, the Foreign Office said it was aware of the incident and had been in close contact with the convoy organisers.
It said its embassy in Tripoli had been in contact with the Libyan authorities and had urged them to resolve the situation.
It added: “Our embassy in Athens has spoken to the shipping company and is also in close contact with the Greek authorities.
“Our priority remains that there be a safe resolution to this incident.”
Ms Merton said the captain became irritable in the port area over payments but the group had given the money in cash to a shipping agent.
The volunteers say they have been searched by Greek commandos She said the captain appeared to fear that the agent would not pay him the £56,000 ($90,000) fee for the trip to Gaza.
Text messages sent from the group to Ms Merton on Thursday as they were taken to Greece suggest that the volunteers were kept in one small cabin.
In one message, Mr Turner said: “Initially we were stopped from moving around the ship and had to stay in the bottom of the lower deck.
“Eventually we just walked through to the crew quarters, and they’ve let us occupy their small smoking room.
“At least we are safe and in the dry, and it has a couch so we can try to sleep. We’ve been given one meal, of sorts, since the ship left Derna. We’ve got access to water, but it’s not drinking water.”
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