US Democratic hopeful Bernie Sanders has hit back after Republican front-runner Donald Trump said he had sent protesters to disrupt his rallies.
The row came after Trump was forced to call off a rally in Chicago on Friday after violent clashes.
Sanders called Trump a "pathological liar".
He and his rival Hillary Clinton also accused Trump of encouraging supporters to beat up protesters and then offering to pay their legal costs.
On Tuesday Republicans and Democrats vote in primaries in states including Florida and Ohio.
Republican candidate Marco Rubio, currently in third place, has said a win in his home state Florida is vital to prevent the Republican nomination being "hijacked" by Trump.
Reacting to Trump's allegations, Sanders also told the billionaire to tell his supporters that violence was not acceptable in the political process.
Meanwhile Clinton accused the Republican of conducting a cynical campaign of hate and fear.
Hundreds of protesters gathered at the venue at the University of Illinois at Chicago on Friday. Inside the auditorium, fighting broke out between supporters and protesters, who waved flags and chanted.
A statement from Trump's campaign said the candidate decided to postpone the event after meeting with police.
However, a Chicago Police Department spokesman said the force had not recommended that Trump postpone the rally.
The clashes began more than an hour before the event was due to start, and continued after it was cancelled, minutes after Trump was to have appeared.
Violent clashes continued outside the venue, with helicopter footage showing chaotic scenes as police attempted to control the large crowds.
Speaking to Fox News after the events, Trump denied using hate speech or playing any part in fostering division.
"I represent a large group of people that have a lot of anger," he said. "There is tremendous anger out there on both sides."
Discussing the decision to cancel the rally, he said: "I think it was a very good thing we did, I think it was an intelligent decision."
Trump's rivals for the Republican nomination, Marco Rubio and Ted Cruz, both called the incident "sad".
A number of Trump rallies have been interrupted by protesters before.
These latest clashes come just a day after a Trump supporter was charged with assault after multiple videos showed him punching a protester at a campaign rally in North Carolina.
The billionaire later said that the supporter's actions were "appropriate".