CAIRO — Egyptian security officials say clashes between police and protesters loyal to the country’s ousted president have killed three people on the first day of a key vote on Egypt’s new charter.

Tuesday’s clashes in the city of Sohag came hours after Egyptians started voting on the new constitution, which represents a key milestone in a military-backed roadmap put in place since the ouster of the Islamist Mohammed Morsi last July.

A massive security operation is underway to prevent Morsi’s supporters from disrupting the vote and in anticipation of attacks by Islamic militants.

The officials say police opened fire on about 300 pro-Morsi protesters after the officers came under fire from rooftops during a rally.

They spoke on condition of anonymity because they were not authorized to talk to media.