The killing of a policeman by a suspected Islamist militant has pushed national security to the top of the French political agenda, two days before the presidential election.

With the first round of voting in the two-stage election taking place on Sunday, far-right nationalist candidate Marine Le Pen promised tougher immigration and border controls to beat "Islamist terrorism" if elected.

Centrist Emmanuel Macron, who narrowly leads a tight race ahead of Le Pen, said the solutions were not as simple as she suggested, and that there was "no such thing as zero risk".

Anyone who said otherwise was irresponsible, said Macron, a former economy minister in the government that Le Pen has repeatedly criticised for its security record.

There are four leading candidates in a race that is still too close to call. Sunday's voting will be followed by a runoff on May 7 between the top two candidates.