(Adds update on Thursday polling; paragraph 4)
MEXICO CITY, Oct 25 (Reuters) - Mexico's incoming government opened polls on Thursday for a public consultation to decide whether to ditch a multi-billion-dollar new airport, a possibility that worries markets and business leaders.
The poll from Oct. 25 to Oct. 28 will show if the government of leftist President-elect Andres Manuel Lopez Obrador should finish the ambitious but costly new Mexico City airport or scrap it and upgrade a military air base to complement a current hub.
The consultation is being held in more than 500 municipalities, home to about 80 percent of the population, incoming transport minister Javier Jimenez Espriu said.
More than 184,000 people cast ballots on Thursday in turnout that exceeded expectations, Lopez Obrador's team said in a statement.
The results of the poll, which will guide the final decision of the new government, are one of the first big tests of Lopez Obrador's economic policy and his relations with the country's business elite he criticized during the campaign.
Moises Kalach, a senior member of Mexico's powerful CCE business lobby, told media he was worried any contract cancellations could violate international treaties, adding that markets needed certainty.
Lopez Obrador, who takes office on Dec. 1 after a landslide victory, jokingly asked reporters to turn off their cameras as he cast his ballot at a Mexico City polling station, saying his vote was secret.
He ruled out an impact on the economy, saying contracts to develop the new airport would be "backed, protected, no injustice will be committed" even if the project was scrapped.
Some business leaders have warned that project cancellation could spook investors and hit the Mexican peso and government bonds.
"The impact on risk premiums and particularly on the currency could be significant," analysts at BBVA Bancomer said in a note.
Opposition parties say the consultation has not followed the rules for referendums.
"This consultation is unnecessary, since it will only serve to justify the decision that the president-elect has already made, and which he made public during his electoral campaign," the leftist Party of the Democratic Revolution said in a statement.
The incoming president has railed against the new airport, saying it has been tainted by corruption and will be costlier than planned.
The new airport, under construction on a drained lakebed on Mexico City's eastern flank since 2015, is about 20 percent to 30 percent complete.
The Texcoco airport is projected to cost 285 billion Mexican pesos ($14.6 billion), up from an original estimate of 195 billion, says incoming transport minister Jimenez Espriu. ($1=19.4640 Mexican pesos) (Reporting by Anthony Esposito, Miguel Angel Gutierrez, Christine Murray and Daina Beth Solomon Editing by Susan Thomas)