PARIS—A gunman opened fire on the Champs-Élysées on Thursday, killing a police officer and wounding two others in an assault authorities said was likely a terror attack, just days before France’s presidential elections begin.

French officials said the assault began at 8:50 p.m., when a car pulled alongside a police patrol and the gunman jumped out wielding an automatic rifle. Police returned fire, killing the gunman, who was identified by an official as Karim Cheurfi, a French national.

A spokeswoman for antiterrorism prosecutors in Paris said they had opened an investigation into the assault. French President François Hollande said authorities were convinced it was a terror attack and expressed “great sadness” over the police officer’s death.

Islamic State claimed responsibility for the suspected terror attack, said SITE Intelligence Group, which monitors the extremist group’s communications. “We can’t exclude whether there’s one or several accomplices,” Pierre-Henry Brandet, the Interior Ministry spokesman said.

The attack sent immediate ripples across the political landscape as the closely fought election was entering its final stretch. France 2, the state TV channel, briefly interrupted a live broadcast in which the 11 presidential candidates were outlining their platforms to broadcast footage showing the Champs-Élysées in lockdown.

“This threat will remain part of daily life for the coming years,” centrist Emmanuel Macron said on the live broadcast as details of the assault began to trickle out. “The first duty, the first mission of the president is to protect.”

The timing and location of the assault, in the shadow of the Arc de Triomphe, was likely to shift the focus of a campaign that has been largely centered on economic issues. A string of attacks—including the Nov. 13, 2015, assault by Islamic State militants that killed 130 in Paris and the truck attack in Nice that killed 86 people on Bastille Day last July—has put France on edge. The government has declared and renewed a state of emergency, but the crackdown hasn’t stopped the drumbeat of periodic attacks.

François Fillon, a conservative who has focused his campaign on countering what he calls “Islamist totalitarianism,” sought to draw contrast with Mr. Macron moments later, saying: “We can’t keep living in this fear, this terror that weighs on the future of the country.”

The shooting unleashed pandemonium along the celebrated shopping thoroughfare as police sealed off the area and police helicopters hovered above, probing the area with searchlights. One foreign tourist was hit by shrapnel, Prosecutor François Molins said.

France’s national police urged Parisians to avoid the surrounding neighborhood, saying an “intervention” was under way.

“Avoid the area and abide by police orders,” French police said in a statement.

Police in tactical gear carrying automatic rifles took up positions along the cobblestoned boulevard. Police cleared the area’s shops, ordering people to evacuate buildings with their hands in the air.

One shopper who was inside a Toyota showroom at the time of the assault said he heard the shots pierce the air “like firecrackers.”

“Everyone started running,” said Rob McKenzie, who was dining inside a pizzeria on the Champs-Élysées when he heard the gunfire. People came pouring into the restaurant seeking cover, said Mr. McKenzie, an Australian in Paris on a business trip.

Police said they were poring over the vehicle used by the assailant to determine whether it contained any explosives.

On Tuesday, authorities detained two men in Marseille on suspicion of plotting an imminent terror attack. Prosecutors said the two men pledged allegiance to Islamic State in a video.

In Washington, U.S. President Donald Trump offered his condolences to the city of Paris.

“What can you say? It just never ends,” he said. “We have to be strong and we have to be vigilant.”