Omar Nazzal, a member of the general secretariat of the Palestinian Journalists Syndicate (PJS), was detained by Israeli officials on Saturday at the border between the occupied West Bank and Jordan.
He was seeking to travel to a meeting of the European Federation of Journalists in Bosnia.
The Shin Bet service said in response to an AFP query that he had been recently appointed a director of Palestine Today, a TV channel it declared illegal in February, and was active in the Popular Front for the Liberation of Palestine -- a leftist political party Israel accuses of terrorism.
It said he was not detained because of his journalism but over "his involvement in terror group activities".
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The PJS called yesterday for the International Committee of the Red Cross (ICRC) to intervene in Nazzal's case.
Union officials met an ICRC representative yesterday in Ramallah in the occupied West Bank, as journalists held a sit-in protest over the arrest.
"We explained to the ICRC that it was imperative to quickly visit Omar Nazzal," Musa al-Sher, general secretariat member, told AFP.
Nazzal's wife told AFP she did not know where he is being held.
"I have no idea of his fate since he told me by phone Saturday he was arrested by Israel," she said.
The Palestine Today offices in Ramallah were raided in March by soldiers after Israel accused it of incitement.
The Palestinian information ministry yesterday again accused Israel of targeting Palestinian journalists, sating that several local radio stations had also been raided in recent months.
In March, the International Federation of Journalists said it was "very concerned" by a wave of arrests of Palestinian journalists.