In a new report the Big Bots Protect, which describes itself as a campaigner against social media abuse, said posts used "lies, libel and rumormongering" to attack Netanyahu's challengers.
It found over 130,000 tweets from "hundreds of fake or anonymous accounts" without names or profile pictures, which did not identify themselves as linked to Netanyahu's Likud party.
It acknowledged that there were also "hundreds of genuine accounts" backing the premier's bid for re-election.
Among the targets were journalists and public figures considered hostile to Netanyahu including his main challenger, Benny Gantz, leader of the centrist Blue and White party.
The report said they included accusations that Gantz was a "rapist" and others questioning his mental health.
A spokeswoman for Blue and White told AFP that party leaders had filed a complaint against the posts with police.
"There is an attempt to steal the elections, there is a system of lies here," Gantz wrote on Twitter.
"Netanyahu's house of cards is collapsing."
The prime minister went live on Facebook and YouTube to dismiss the report, flanked by what he said was a genuine Likud online supporter.
"A million Likud voters are not 'bots'," he said, adding that Likud was also filing a police complaint against Blue and White for accusing him.
"Not one of them is fake," he said of the messages cited by the Big Bots Protect.
"They have names, they have families," he added. "They have their own opinions, independent people!"
Netanyahu, 69, is seeking a fifth term in office in the April 9 election, despite also facing potential corruption charges.
Blue and White has consistently been scoring better than Likud in opinion polls, although Netanyahu is seen as standing a better chance of putting together a coalition government under Israel's proportional representation electoral system.
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