One policeman has been shot dead and two others wounded in central Paris, French police say, with their suspected attacker killed by security forces.
A lone gunman opened fire before being killed as he fled the scene, police say. The Champs-Elysees was sealed off.
President Francois Hollande said that he was convinced the attack was "terrorist-related".
So-called Islamic State (IS) said that one of its "fighters" had carried out the attack.
Islamist militancy is a major issue in the polls after recent mass attacks claimed by IS, with 238 people killed in jihadist attacks in France since 2015, according to data from AFP news agency.
Several candidates in Sunday's presidential election have ended their campaigns early as a mark of respect, with the centre-right's François Fillon calling on others to halt theirs too.
"On the face of it, the officers were deliberately targeted," Interior Ministry spokesman Pierre-Henry Brandet said.
Hollande said after crisis talks with his prime minister and interior minister that the security forces had the full support of the nation and a national tribute would be paid to the fallen policeman.
He said he would hold a meeting of his security cabinet on Friday morning.
Because of its worldwide renown and its large number of visitors, the Champs-Elysees has long been seen as a potential target, the BBC's Hugh Schofield reports from the French capital.
A car pulled up alongside a police bus just before 21:00 (19:00 GMT) and a man got out, opening fire with an automatic weapon at the bus, Mr Brandet said.
After killing an officer, he attempted to run away while shooting at other officers, two of whom he injured, the spokesman added.
He was then shot dead by security forces.
Paris prosecutor Francois Molins later said that "the attacker's identity is known and has been verified".
"I won't reveal it, because investigations and raids are already under way, in particular to establish whether there is any evidence or not of complicity (in this attack)."
Meanwhile, IS identified the attacker as Abu-Yusuf al-Baljiki, in a statement carried by its Amaq news outlet.