* Macron seeks to quell doubts over experience, backing
* Macron and Le Pen are still the front-runners
* Prosecutors refuse to open inquiry into Hollande (Updates with Macron interview, poll)
By Adrian Croft and Jean-Baptiste Vey
PARIS, April 6 (Reuters) - French centrist Emmanuel Macron sought to establish his presidential credentials on Thursday by calling for international military intervention against Syrian President Bashar al-Assad if he is proved to have used chemical weapons.
The former economy minister is favourite to win France's presidential election but faces criticism from his opponents that he is too inexperienced for the top job.
Macron, 39, interviewed for 2-1/2 hours on France 2 television, took a tough stance on the top foreign policy issue of the day, calling for military intervention against Assad if his government was found to have carried out a suspected chemical attack that killed at least 70 people on Tuesday.
"An international intervention is needed ... My preference is that there should be an intervention under the auspices of the United Nations. A military intervention," Macron said.
However, he said a military operation must be part of a "diplomatic and political roadmap."
Russia has repeatedly used its veto in the past to protect the Syrian government from U.N. Security Council action.
Polls show Macron and far-right leader Marine Le Pen each winning about a quarter of the vote in the April 23 first round of the election, with Macron tipped to easily defeat Le Pen in the May 7 runoff.
Scandal-hit conservative candidate Francois Fillon is holding on to third place in the latest polls, followed closely by 65-year-old Jean-Luc Melenchon, a veteran far-left maverick, who has moved up fast in the ratings.
Macron, a pro-European former banker who has never held elected office, aims to transcend the traditional left-right divide in French politics and boost economic recovery by reducing public spending and cutting taxes.
Le Pen wants to curb immigration, ditch the euro and bring back the franc, and hold a referendum on European Union membership.
CANDIDATES
Macron sought to answer questions on Thursday about how he would govern, if elected, since the En Marche! (Onwards!) political movement he created a year ago has never taken part in an election.
That has prompted opponents to cast doubt on his ability to win a majority in the lower-house National Assembly, which would hamper his ability to push through planned reforms.
Macron presented on Thursday the first 14 candidates who will represent his party in June legislative elections, headed by Jean-Michel Fauvergue, 60, a former director of an elite police counter-terrorism unit.