The editor of a daily Irish language newspaper has said the Northern Ireland man held by Israeli police was working as their correspondent.
Seán O Muireagáin is being detained by Israeli security forces
The man - who is in his 40s - was arrested at the weekend by Israeli security forces reportedly acting on information from UK security services.
The west Belfast man was arrested near the town of Ramallah.
Ciaran O Pronntaigh of the Belfast paper, Lá, said the allegations made against Seán O Muireagáin were "rubbish".
"He is a well-known Irish language and Palestine Solidarity Committee activist in Belfast and was reporting from Jenin for Lá and Raidió na Gaeltachta. He was arrested on his way into Ramallah," he said.
Mr O Pronntaigh said the paper had contacted the Israeli Embassy in Dublin to demand his immediate release.
He added: "On July 8, we published a full-page account from Jenin in the West Bank by Seán O Muireagáin in our paper, a piece which was accompanied by his photograph, proof that he was in the occupied territories openly.
"We have contacted the Israeli Embassy in Dublin to demand Mr O Muireagáín's immediate release."
'Worried for safety'
Earlier, a Palestinian support group based in Belfast said the Northern Ireland man was in the West Bank on a cultural exchange project.
The Ireland Palestine Solidarity Campaign said it last heard from him on Saturday and was worried for his safety.
Kathleen Connell from that group said: "The thing that's giving us cause for great concern is the fact that his family have been trying to contact the British Embassy in Israel who are not answering the phone."
The Associated Press news agency said the man was arrested on Saturday afternoon and was being questioned by the Sin Bet security agency.