In another rejection of diplomatic convention, US President Donald Trump has inserted himself into the French presidential race by tweeting that the latest Islamist attack in Paris will have a “big effect” on voters there.

“Another terrorist attack in Paris. The people of France will not take much more of this. Will have a big effect on presidential election!” he tweeted.

His tweet came as Mr Trump tries to bolster US support for hard-line proposals on immigration and foreign policy, demanding that Congress fund a bigger US border wall with Mexico and suggesting that the US needs to scrap the nuclear deal with Iran cut by White House predecesor Barack Obama.

On Friday (AEDT) , the Paris attacker used an automatic weapon to shoot officers in the centre of the Champs-Elysees. One gendarme was killed, as was the gunman. The Islamic State claimed responsibility.

The outcome of France’s presidential election is being closely watched for signs that Europe is moving toward nationalist candidates who advocate the European Union’s dissolution. The top two candidates from Sunday’s vote in Paris will progress to a winner-takes-all May 7 runoff.

The policeman’s killer was detained by police in February after a counter-terrorism raid, but later released, French media has reported.

Islamic State has claimed responsibility for the Paris attack, on the emblematic Champs-Elysées avenue.

In a statement via the group’s Amaq news agency, IS identified the attacker as one of its soldiers — who French police said used a “weapon of war” — naming him as Abu Yousif, the Belgian.

However, French newspaper Le Parisien reported that according to sources close to the investigation, the killer was a 39-year-old Frenchman, who had been the subject of a previous counter-terrorist investigation.

Le Parisien reported the suspect was known as Karim C. While police confirmed they knew his identity, they declined to name him.

“That, therefore, begs the question over whether this is the same man as that given by Daech (IS),” the paper asked.

The gunman had been detained in February for questioning in a police station in Meaux, outside Paris, after “informants” had indicated that he was “seeking to obtain weapons to kill policemen”, according to media reports.

But due to a lack of evidence, the man was eventually released.

Le Parisien further reported that the suspect was convicted in 2003 for the attempted murder of three men, with his sentence eventually reduced to five years in 2005.

Paris police spokeswoman Johanna Primevert told reporters the attacker targeted police guarding the area near the Franklin D Roosevelt metro stop on Thursday night (local time) at the centre of the avenue, which is popular with tourists.

French Interior Ministry spokesman Pierre-Henry Brandet said police officers were deliberately targeted in the shooting, but it is too early to say what the motive was.

Mr Brandet said apart from the policeman killed on the spot by the attacker, two police officers had been seriously wounded.

“An automatic weapon was used against police, a weapon of war,” he said, adding that the “terrorist threat” in the country remained high.

French President Francois Hollande said the attack was “likely to be terrorism”, that the nation’s security was at stake and he convened a crisis meeting.

“Our security forces, our police, our military police will remain on the highest state of alert as has been the case for a few months,” he said.

Prime Minister Malcolm Turnbull said Australia’s prayers were with the family of the policeman, saying the attack had all of the hallmarks of a terror attack.

“But at this stage I’m advised that the French prosecutor is on the scene and they’ll obviously be making further announcements as they investigate the crime,” he told the Seven Network on Friday.

Mr Turnbull urged Australians to pay close attention to travel advisories from foreign affairs officials.

“Everywhere – but especially in Europe at the moment – pay close attention to your surroundings,” he said.

Police car attacked at traffic lights

Mr Brandet said the shooting started at shortly after 9pm local time when a car stopped alongside a police car at a red light.

“A man immediately got out and opened fire on the police car, fatally wounding a police officer,” he said.

“He also wounded a second one, it would seem very seriously.”

Police authorities called on the public to avoid the area.

France takes to the polls on Sunday in the first round of a closely fought presidential election.

France has lived under a state of emergency since 2015 and has suffered a spate of Islamist militant attacks that have killed more than 230 people in the past two years.

Earlier this week, two men were arrested in Marseille whom police said had been planning an attack ahead of the election.

A machine gun, two hand guns and three kilograms of TATP explosive were among the weapons found at a flat in the southern city along with jihadist propaganda materials according to the Paris prosecutor.

— with ABC/AAP