Several hundred people were protesting against a proposed UN-backed peace deal that would form a new unity government.
The mortars were fired from an Islamist-held district, a police spokesman told AP news agency.
Over the last year, Benghazi has seen fierce clashes between the military and various Islamist brigades.
Among them are Ansar al-Sharia and groups that have pledged allegiance to the so-called Islamic State militant group.
Friday’s protest central Benghazi was against proposals, brokered by the United Nations envoy to Libya, for a new unity government.
The proposal was made after nearly a year of talks between the Islamist-backed parliament in Tripoli and the internationally recognised administration in the east.
Both parliaments will have to vote on whether to accept the deal but many MPs across the country have dismissed the plan.