Riots could break out in cities across France after results are announced of the first round of the presidential election, intelligence services have warned, as the most unpredictable vote in decades goes ahead amid a heightened terror alert.

Trouble is almost certain if the far-Right leader Marine Le Pen and her far-Left counterpart Jean-Luc-Mélenchon are the two candidates who make it through to the second and final round on May 7, according to a report issued by French intelligence services.

The confidential document, leaked to Le Parisien newspaper, said that at the top of the list of potential security problems as millions of France cast their ballots was the “jihadist threat.”

The report came just two days after a French jihadist claiming allegiance to the Islamic State (Isil) shot dead a policemen on the Champs Elysées avenue in Paris, bringing election campaigning to an early end and thrusting security issues back to the top of the political agenda.

Around 50,000 police officers and 7,000 soldiers will be deployed to protect voters around France on Sunday for the first round, which has turned into a four-way race between Ms Le Pen, Mr Mélenchon, the scandal-scarred conservative François Fillon, and the maverick centrist Emmanuel Macron.

An opinion poll conducted on Thursday and Friday showed Ms Le Pen and Mr Macron tied on 23 percent, ahead of Mr Mélenchon with 19.5 per cent and Mr Fillon on 19 per cent.