The Islamic State, through its affiliated Amaq news agency, quickly claimed responsibility for the attack, which sent panicked pedestrians fleeing into side streets and prompted police to seal off the renowned Champs-Elysee, close metro stations and order tourists back into their hotels. The terrorist organization said the attack was carried out by a Belgian national it identified only as Abu Yusuf al-Baljiki, a pseudonym.
There was no immediate confirmation that the Islamic State was behind the shooting. French officials declined to attach a motive to the attack, although they said police were deliberately targeted and that they were opening a terrorism investigation.
A European security official told The Washington Post that the dead attacker was known to French intelligence, having previously come to authorities' attention for radical Islamist links. Two officials said the gunman used an AK-47 assault rifle in the attack.
The French Interior Ministry said one police officer was killed on the spot and two others were "seriously wounded" when the gunman opened fire on a police car. The ministry said security forces gunned down the attacker as he tried to flee the scene on foot.
A spokeswoman for the Paris police, Johanna Primevert, said the gunman attacked police guarding an area near the Franklin Roosevelt metro station Thursday night at the center of the heavily traveled Champs-Elysees.
She said the attacker appeared to act alone, but other officials said it was too soon to tell whether he might have had an accomplice.
The Reuters news agency reported that police issued an arrest warrant for a second suspect who they said had arrived in France by train from Belgium.
Interior Ministry spokesman Pierre-Henry Brandet told France's BFM television that the gunman got out of a car that pulled up beside a police vehicle and opened fire on the police officers.
"It's too early to say what's behind this, but clearly police were the target," he said. "We don't know yet what his motivations were." There were conflicting reports about whether another person was in the gunman's car.
In Washington, President Donald Trump said during a news conference with the visiting Italian prime minister that the Paris shooting "looks like another terrorist attack," and he offered condolences to France.
"Again it's happening, it seems," Trump said. "I just saw it as I was walking in. ... That's a very, very terrible thing that's going on in the world today. But it looks like another terrorist attack. And what can you say? It just never ends. We have to be strong and we have to be vigilant, and I've been saying it for a long time."
The incident occurred three days before France holds the first round of a hotly contested presidential election, with candidates from across the political spectrum vying to succeed François Hollande as president. Hollande scheduled an emergency meeting late Thursday to discuss the attack.
The country has been hit by a deadly wave of terrorist violence in the last two years that has claimed the lives of at least 230 people and injured hundreds of others.
Another mass-casualty terrorist attack occurred last July in the French Riviera city of Nice, when a Tunisian resident of France drove a cargo truck through crowds of revelers celebrating Bastille Day, killing 86 people. The Islamic State claimed responsibility.
More recently, French soldiers were targeted in two separate attacks in February and March as they helped to provide security at the Louvre museum and Orly airport.
Thursday's shooting - on the most famous boulevard in the French capital, always crowded with tourists and commuters - came just two days after authorities arrested two men in the southern city of Marseille on suspicion of plotting what Paris prosecutors described as an "imminent" and "violent" assault. Police discovered an Islamic State flag and three kilograms (6.6 pounds) of explosives in one suspect's home.
The Islamic State has claimed responsibility for previous attacks in France, including a coordinated November 2015 terrorist assault on multiple targets in Paris that left 130 people dead and more than 360 wounded.
After that attack and others in the last two years - many perpetrated by Islamic State militants or those claiming to be inspired by the extremist group - terrorism and national security remain crucial issues in the most contentious election France has seen in decades.
The leader of the far-right National Front party, Marine Le Pen, has campaigned heavily on an anti-immigrant platform and what she has couched as the need to defend France from "Islamist globalization." In the final days of the campaign, she said she would halt immigration altogether if elected president.
The shooting occurred in the middle of a televised campaign event, when each of the 11 current candidates was given 15 minutes to sell voters on their respective platforms.
There was no immediate information on the identities of the attacker or the policemen who were shot.
According to Christophe Crepin, a spokesman for the UNSA Police Union, the gunman opened fire on the police with an AK-47 assault rifle, targeting officers who were near a Marks and Spencer store on the corner of the busy avenue. Crepin said one man carried out the attack but that others could have been involved.
Police ordered people away from the area, and at least three metro stations were closed, the Interior Ministry said.
Branigin reported from Washington. Souad Mekhennet in Frankfurt, Germany, contributed to this report.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)