There has been growing concern in Cairo about a decree issued by President Mohamed Morsi that gives Egypt's armed forces powers of arrest and detainment during Saturday's constitutional referendum vote.

The decree, which lasts until the result of the referendum is announced, has reignited the issue of detainment of civilians in Egypt and their subjection to military trials. During the transitional period overseen by the military junta, some 12,000 civilians were tried and sentenced in military tribunals.

Amnesty International called the decree a "dangerous loophole" that could once again lead to detainment of civilians. "Considering the track record of the army while they were in charge, with more than 120 protesters killed and in excess of 12,000 civilians unfairly tried before military courts, this sets a dangerous precedent," said Amnesty's deputy Middle East and north Africa director, Hassiba Hadj Sahraoui.

The constitutional referendum comes at a polarised time in Egyptian politics, after Morsi issued a decree granting himself extraordinary powers, which he partially rescinded on Saturday after protests. The opposition National Salvation Front (NSF) has rejected the referendum and called on Egyptians to stage peaceful protests on Tuesday .

Opposition forces were considering whether to boycott the referendum or mobilise for a "no" vote if it is not postponed. NSF member Hassan Moanis told Associated Press: "For now, we reject the referendum as part of our rejection of the draft constitution."

Morsi also suspended a series of tax rises on Monday morning, hours after they had been issued. The increases were announced as part of an economic reform package being introduced before a 19 December deadline for International Monetary Fund approval of a $4.8bn loan. Then at 2am local time, a message was posted on Morsi's official Facebook page suspending – not cancelling – the tax hikes because he "felt the pulse of the streets and is aware of how much the Egyptian citizen is burdened in these tough economic times".