Israel's expulsion of French-Palestinian human rights lawyer Salah Hamouri amounts to a "war crime", the UN human rights office said on Monday.
Mr Hamouri, 37, arrived in France on Sunday after being held without charge in Israel under a controversial practice that allows suspects to be detained for renewable periods of up to six months.
"Deporting a protected person from occupied territory is a grave breach of the Fourth Geneva Convention, constituting a war crime," UN human rights spokesman Jeremy Laurence said in a statement.
Israel has accused Mr Hamouri of being a member of the Popular Front for the Liberation of Palestine (PFLP) and of representing a security threat. Mr Hamouri denies links to the PFLP.
In condemning his expulsion, Mr Laurence said: "We are deeply concerned by the chilling message this sends to those working on human rights" in occupied East Jerusalem.
The French Foreign Ministry has also criticised the deportation. Paris had been "fully mobilised, including at the highest level of the state", to enable Mr Hamouri to defend his rights, benefit from all possible assistance and lead a normal life in his native East Jerusalem, a statement read.
"France also took several steps to communicate to the Israeli authorities in the clearest way its opposition to this expulsion of a Palestinian resident of East Jerusalem, an occupied territory under the Fourth Geneva Convention," it added.
Born in East Jerusalem, Mr Hamouri does not have Israeli citizenship but held a residency permit that Israeli authorities revoked.
"We didn't think it was possible to deport somebody from his birthplace," Mr Hamouri's mother Denise has said.
Israel has occupied the West Bank and East Jerusalem since the 1967 Six-Day War.