Voting is under way in Ecuador's second-round presidential runoff between leftist former vice president Lenin Moreno and conservative former banker Guillermo Lasso.

Opinion polling ahead of Sunday's vote showed Moreno and his running mate, current Vice President Jorge Glas, with a four-point lead over Lasso and his running mate Andres Paez, backed by the centre-right alliance CREO-SUMA.

Moreno, the candidate of the ruling PAIS alliance and the hand-picked successor to outgoing President Rafael Correa, won 39.36 percent of the vote in the election's first round on February 19, falling just short of the 40 percent and 10-point-lead necessary to win outright.

Lasso won 28.9 percent of the first round's votes, but was expected to pick up more votes after conservative Congresswoman Cynthia Viteri, who finished third in the first round, threw her support behind him.

Lasso, 61, a director of the country's largest bank, Banco de Guayaquil, served previously as finance minister, provincial governor and US ambassador in the government of former president Jamil Mahuad.

Moreno, 64, served as Correa's vice president until 2013, when he accepted an appointment as United Nations special envoy on disability. The author and former professor uses a wheelchair after a shooting in 1998 left him paraplegic.

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The election is seen not only as a referendum on the two candidates but on popular satisfaction with Correa's decade-long leftist rule.

Correa remains popular, but recent changes of guard in South American countries including Argentina, Brazil and Peru have shown the continent tacking to the right.

Some 12.8 million Ecuadorians are eligible to vote at 40,000 polling places. Polls opened at 7am (12:00 GMT) and will close at 5pm (22:00 GMT).