Friends and family of an aid volunteer from Bolton, travelling on the Gaza-bound ship targeted by Israeli forces, have said they fear for his life.
Adam Zanghar, who is married with children, was accompanying £32,000 of aid paid for by Bolton people.
He was among 500 Palestinian aid workers on board the ship, which was stormed by Israeli commandos who boarded by helicopter overnight.
Vigils and protests were being held in Preston, Bolton and Manchester.
Nine people have been confirmed dead by the Israeli authorities.
A friend of Mr Zanghar, Salim Bhorat, said: "We are very worried about Adam and all the people on board.
"His family are very distressed and they are trying to get information, but we haven't heard from Adam in the last 24 hours and we are just hoping that the silence is because of lack of phone coverage in the area they are in."
Mr Bhorat said that the aid donated by Bolton people had been contributed from street collections and faith and community groups.
"The money was used to buy building materials, medicines and food to help people in Gaza," he said.
A vigil was held at Victoria Square in Bolton at 1700 BST to remember those killed in the incident.
Chair of Manchester Palestinian Solidarity Campaign, Linda Clair said: "Our national director Sarah Colborne is on board as well and we have heard that 40 British people were on the ship and have no idea if they are among the dead or injured.
The flotilla has been directed towards the Israeli port of Ashdod, where passengers will processed by Israeli officials.