Francois Fillon won France's first-ever conservative presidential primary on Sunday after promising drastic free-market reforms and a crackdown on immigration and Islamic extremism, beating a more moderate rival who had warned of encroaching populism.

"President! President!" chanted the former prime minister's supporters as he declared victory over Alain Juppe in a nationwide run-off election.

Polls suggest Fillon, 62, would have a strong chance of winning the French presidency in the April-May election, amid widespread frustration with France's current Socialist leadership.