JERUSALEM — As Palestinian prisoners ended the third week of a mass hunger strike, Israel released videos that it said showed the strike leader, Marwan Barghouti, sneaking snacks in his cell.

Palestinian leaders dismissed the videos, released on Sunday, as fakes aimed at deflating the strike of over 1,000 prisoners, which has stirred protests and clashes with security forces in Palestinian areas. The prisoners are demanding better conditions in Israeli jails.

Prison officials said the videos showed Mr. Barghouti, the strike’s organizer, eating cookies on April 27 and then a candy bar on Friday. He is being held in solitary confinement at the Kishon prison.

Israel’s public security minister, Gilad Erdan, said the strike was not about prison conditions, which he said exceeded international standards, but about Mr. Barghouti’s political goals. Though serving life sentences for five murders during the Second Intifada of the early 2000s, Mr. Barghouti, 57, is popular among Palestinians and is seen as a possible successor to the unpopular Palestinian Authority leader, Mahmoud Abbas, 82.