Voting is obligatory in Ecuador so queues formed early for the referendum which asked voters to decide on measures aimed at overhauling the political system.
Specifically, those voting had to decide if they backed setting up a people's assembly to re-write the constitution.
The reforms aim to improve the lives of the poorest people in Ecuador, according to President Rafael Correa.
Mr Correa exuded confidence as he arrived to cast his vote - opinion polls suggested he would win.
The "No" camp and his critics argue it is a disastrous step aimed more at cementing Mr Correa's hold on power.
Ecuadoreans are used to political turmoil, having seen eight presidents in the last 10 years.
Whatever the outcome, Ecuador faces huge challenges in improving the lives of its poor majority.