"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 said, breaking a silence over Sunday's vote in a tweet.
Trump tweeted hours after a gunman shot dead a policeman and wounded two others on the world-famous Champs Elysees boulevard.
The attack rocked France's presidential race Friday with just days to go before one of the closest races in recent memory.
Bloodshed had long been feared ahead of Sunday's first round of voting after a string of jihadist atrocities since 2015, and the shooting on the world-renowned boulevard forced security to the top of the agenda in the campaign.
France is in a state of emergency and at its highest possible level of alert since a string of terror attacks that began in 2015, which have killed more than 230 people.
The vote is being closely watched as a pivotal event amid a global swing to the right, including Trump's election in November and Britain's decision to exit the European Union last summer, a move the US president has applauded.
Macron, a 39-year-old moderate, is presenting himself as a guarantor of the EU, promising reform and appealing to pro-Europe sentiment among the French electorate.
Le Pen, widely seen as taking the hardest line on security, has called for France to "immediately" take back control of its own borders from the European Union and deport all foreigners on a terror watchlist.
Barack Obama spoke to Macron on Thursday about the "important upcoming presidential election in France," a spokesman for the former US president said.
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