A slim majority of Turkish voters agreed on Sunday to grant sweeping powers to their president, in a watershed moment that the country's opposition fears may cement a system of authoritarian rule within one of the critical power brokers of the Middle East.

With nearly 99 per cent of votes in a referendum counted on Sunday night, supporters of the proposal had 51.3 per cent of votes cast, and opponents had 48.7 per cent, the country's electoral commission announced.