If a government has absolute power, there is always a risk of it becoming tyrannical, dictatorial and oppressive, resulting in human rights violations, revolts or violence. In pure democracy, the parliament is supreme and has unlimited power while in a republic like USA the constitution is supreme and the powers of the congress are limited by the constitution. In an Islamic Republic, the Word of God is supreme and the powers of the Islamic government are limited by the Quran or a constitution based on the Quran.
According to the Quran, “No human being – even though God may have given him a Book (containing the Code of Laws) or the power to enforce it or even Nubuwwat (prophet status) – has the right to say to the others: ‘You should serve me rather than God,’ what he should say is: ‘You should be amongst those who belong to God by following His Book which you teach to others and study yourself”. In an Islamic Republic, all judgments, decisions and commands are according to Allah’s revealed Book and nothing can be against or repugnant to it.
One check on the Islamic Government is that there can be no legislation or decisions against the Supreme Law enshrined in Allah’s Book. Another check is that all the subsidiary laws and decisions should be made by mutual consultation. Even the authority of the Prophet (s.a.w) was limited by these two restrictions.