The government of Venezuela's President Nicolas Maduro, whose 2018 election is not recognized by part of the international community, won a landslide victory in Sunday's regional ballot, according to election officials.
Candidates aligned with Maduro have won 20 of the 23 governor posts and the mayorship of the capital Caracas in the face of a divided opposition running in an election for the first time in three years.
The mission will present a report on Tuesday.
Sunday's turnout was 41.8 percent, according to official provisional results after 90.21 percent of ballots were counted.
The opposition won three states including oil-rich Zulia, the country's most populous region whose capital Maracaibo is Venezuela's second-largest city.
"Beautiful triumph, beautiful victory," Maduro said of the results.
Before the announcement, opposition figure Henrique Capriles expressed reservations about the late closure of polling stations facilitating fraud.