French prosecutors have launched a probe into whether "fake news" - allegedly with Russian links - was being used to seek to torpedo front-runner Emmanuel Macron's presidential campaign ahead of Sunday's run-off.

The preliminary investigation was opened after Mr Macron sued his far-Right rival Marine Le Pen for implying he had an "offshore account in the Bahamas".

Marine Le Pen and Emmanuel Macron faced off in a televised debate on Wednesday. Credit:AP

It came as Barack Obama, the former American president, endorsed Mr Macron for president, saying he represented "liberal values" and "appeals to people's hopes not their fears".

Mr Macron's legal complaint referred to comments Ms Le Pen made in Wednesday night's TV debate between the anti-EU, anti-immigration candidate and her liberal, pro-EU rival, widely dubbed the most brutal in French presidential history.