Supporters of banned speaker Sheikh Raed Salah are planning to protest against his detention at Downing Street later today.
The Palestinian Forum, which was due to host Mr Salah at events in Brent and Manchester over the weekend, said it wanted to express anger at "the British government's attempt to silence" him.
Sheikh Salah, leader of the northern branch of the Islamic Movement in Israel, has been described in Parliament as having a "history of virulent antisemitism".
He entered Britain last week and spoke at three events in London and Leicester despite being banned.
He was detained on Tuesday evening and the following day the Home Secretary issued him with a deportation order, which his lawyer said he would appeal before the July 6 deadline.
According to the Palestinian Forum, Sheikh Salah has now been moved from a detention centre to a formal prison, a move which it said would "significantly interfere" with his appeal.
The demonstration has been organised with the support of the Palestinian Solidarity Campaign, the Stop the War Coalition and the Friends of Al-Aqsa group.
It has now emerged that Sheikh Salah was able to enter Britain without being stopped, despite the ban, because border officials believed he was flying to Terminal One rather than Terminal Five and sent an alert to the wrong place.
A UK Border Agency source told the Daily Telegraph that "there were a series of cock-ups in terms of getting information to the front-line".
Dr Hanan Chehata, from the pro-Palestinian Middle East Monitor, accused "the pro-Israel lobby in the UK" of targeting Sheikh Salah with a "vicious and concerted smear campaign".
He wrote in a New Statesman piece: "Unfortunately our government has now weighed in to give legitimacy to the systematic persecution of Palestinians on British soil as well."