Talks between Intel and ultra-Orthodox Jews broke down earlier this week
Ultra-Orthodox Jews in Jerusalem have protested outside the offices of the US firm, Intel, against the plant operating on the Jewish day of rest.
The demonstrators chanted "Shabbes! Shabbes!", the Yiddish word for Sabbath when Jews are forbidden to work.
Intel, the world's biggest maker of computer chips, ringed its offices with barbed wire before the protest. There were no reports of violence.
Ultra-Orthodox Jews have recently held a series of such protests in Israel.
More than 1,000 people took part in Saturday's rally.
It was staged after talks between Intel and Jerusalem's ultra-Orthodox Jewish community broke down earlier this week.
Intel has been operating on Saturdays for years - and the company defends the policy.
"We have always worked according to the company's needs. If the needs call for it, we work on the Sabbath as well," Intel Israel spokesman Kobi Becker told Israel's Ynet website.
Israel's ultra-Orthodox Jews have become increasingly active in recent months in their attempts to enforce a Saturday work ban.
They have held regular protests against the opening of a Jerusalem car park on the Jewish Sabbath.