The opposition is expected to control between 23 and 30 seats with the rest going to centrist and indigenous groups, according to exit polls.
Following the preliminary results, Correa said his supporters had achieved "a historic victory" but gave assurance that the 130-member assembly would not seek totalitarian powers and would call for dialogue with the opposition.
But some ministers and candidates have given mixed signals on reforms they want.
The assembly will debate a draft of constitutional reforms put together by academics, and a final version must be approved in a popular referendum after at least six months.
The strong majority should allow Correa's party to control the assembly and push to eliminate the power of the political elite he blames for Ecuador's corruption and instability.
|Critics say Correa, left, wants to change the |
constitution to consolidate power [AP]
Lenin Moreno, the vice-president, urged the opposition to accept the people's decision.
But a fragmented opposition has vowed to stop Correa from using the assembly to consolidate presidential powers and tighten his grip on key state institutions.
In pushing for reforms, Correa has said: "It's necessary to do away with the myths of neo-liberalism."
He called for "21st century socialism" with guarantees for education and free healthcare, but ruled out nationalising private companies.