Washington: US President Barack Obama said on Saturday that political leaders "must speak out against violence", in an apparent reference to disturbances in and outside a campaign event venue in Chicago involving supporters and opponents of Republican front-runner Donald Trump. "If they refuse to do that, they don't deserve our support," Obama said at a Democratic Party fundraising event in Dallas. Politics at its best is a "battle of ideas" that does not involve promoting or resorting to violence, Obama said. "This is not about political correctness. It's about not having to explain to our kids why our politics sounds like a schoolyard fight. We shouldn't be afraid to take them to rallies or let them watch debates." He made his remarks after five people were arrested and two policemen were injured in altercations at the Trump campaign event venue on Friday night in Chicago. Neither of the two injured police was seriously hurt, although one was struck in the head with a bottle. Trump was forced to postpone the campaign event and move it to another date due to the presence of hundreds of demonstrators inside the University of Illinois at Chicago Pavilion. Those protesters celebrated the news, and scuffles then broke out between the two opposing sides both inside and outside the venue. A coalition of the city's activists, students, and religious and political figures, including numerous Latino and African-American organisations, had begun organising the protests days earlier. Trump said on Saturday that his campaign had been the target of a "planned attack" and accused supporters of Democratic presidential candidate Bernie Sanders of instigating the violence. "They want me to tell my people please be nice be nice. My people are nice. They were taunted and harassed by these other people," Trump said at a campaign appearance in Ohio, where a man was tackled by Secret Service agents when he tried to storm the stage while the candidate was speaking. Trump's main rival for the GOP nomination, Texas Senator Ted Cruz, said the tough-talking real-estate mogul was partly to blame for the incident in Chicago, saying it was a "predictable consequence" of the candidate's encouragement of violence against demonstrators. Florida Senator Marco Rubio, meanwhile, said on Saturday that the idea of supporting Trump should he become the Republican nominee was becoming "harder every day", alluding to the negative discourse he said Trump had fostered.