CAIRO (Reuters) - Egypt's Justice Minister Ahmed Mekky has resigned in protest at what he sees as "an assault" on the judiciary by the Islamist-led authorities, the Justice Ministry spokesman said on Sunday.

Mekky submitted his resignation to President Mohamed Mursi on Saturday, said the spokesman, Ahmed Salam. It followed a protest on Friday by Mursi's Islamist backers demanding the "purification" of the judiciary.

A bill has been submitted to parliament which critics say would give the government too much control over the make-up of the judiciary.

Mekky, who was appointed in August, had been quoted in local media as saying he would resign if the law was passed. The Islamist-dominated upper house of parliament has yet to vote on the legislation.

Mursi said on Saturday he planned to reshuffle the cabinet.

Mekky was widely regarded as a supporter of judicial independence during the rule of former President Hosni Mubarak, but his critics had accused him of siding with the new Islamist-led authorities.

Thousands of Islamist supporters of Mursi rallied on Friday, calling for an independent judiciary by passing the new legislation and removing corrupt members of legal institutions.

Friction between the government and the judiciary has been on the rise since March when the Administrative Court ordered the cancellation of a Mursi decree calling for parliamentary elections, forcing a delay in voting due to have begun in April.

(Writing by Tom Perry and Asma Alsharif; Editing by Janet Lawrence)